The Sentinel Berserker, more commonly called the Angry Sentinel tactic, was a Sentinel 4th Edition Imperial Guard tactic that was rather unique - not in that it was especially Cheesey, but because it was good at screwing over horde armies. If done right, the purpose of the tactic was to give your opponents a choice of damnations: to either ignore a low-cost expendable unit that has every ability to do some good damage if it gets stuck in, or to focus it down and shoot it to death while allowing other Guard units to continue their part in the operation - the Sentinel outfitted the way it was being just enough of a threat to warrant keeping an eye on.
How it Worked
Back in the 4th Edition Imperial Guard codex, one option available to players was Regimental Doctrines. These could give a number of boons to your units, like new equipment, stat boosts, or even new special rules, albeit at the cost of losing access to different units. One of the more entertaining Regimental Doctrines was Hardened Fighters, which could be given to Guard Infantry units or Sentinels, at +15 points for the Infantry units and +10 for Sentinels. The doctrine was very simple: +1 to WS for infantry units and +1 to WS and Attacks for Sentinels.
Presumably it was included to help reliably portray the Catachan Jungle Fighters now that their codex had rotated out.
The tactic begins with giving the doctrine to a Sentinel and fitting it with a Heavy Flamer. For 50 points, you'd have a comparatively fragile (but fast) unit you could use to immediately charge into the fray against another player's forces. Because it was a relatively fragile vehicle, most wouldn't consider the thing a priority threat until they got in range. Some players, wanting to make them better as a distraction, made them more appealing targets by mounting Hunter-Killer Missiles, which cost only 5 points in this edition and gave them the ability to fire a one-off shot at targets of opportunity, or Extra Armor, to ensure they got where they needed to and cost the same price. Players who wanted the unit to be a bit more threatening and/or survivable could add a second squad member or a second solo Sentinel outfitted this way, and it still wouldn't likely break the bank in even the cheapest games.
Why It Worked
Vehicle rules in the edition this tactic surfaced with were not very kind to infantry in assaults. They could only hit walkers with Grenades on a 6, which ironically made a single Sentinel with its terrible 10 all-around armor (Armored Sentinels were not around in this edition) potentially able to tie up a big squad of Guardsmen, Guardians, or Termagaunts for quite a while, but its WS 4 meant they were relatively hard to hit, and with 2 attacks each at S5, Sentinels were actually halfway decent in close-combat.
If you didn't stop the Sentinels before they got in close, however, they could cause some serious trouble for anything with less than Power Armor. In this edition (and 5th), Vehicles that were assaulted were not considered tied up in an assault - and could continue to open fire on the unit they were assaulting. A single Heavy Flamer is always an unpleasant thing for a squad of light infantry to get hit by, but a Heavy Flamer that you can't get away from once and keeps shooting you while you're locked in combat with the source of it is what made this so hilariously good at terrorizing cover campers.
One major reason this tactic worked as well as it did was that Sentinels in 4th Edition had that edition's variant of the Scouts rule. This meant that you could basically always deploy them with the rest of your army, even on missions when you were only allowed to bring in vehicles later. As a result the things were always allowed to be on the board, making pests of themselves. The optional HK Missile upgrade did a good job of making them more annoying, since a missile on something as mobile as a Sentinel was going to be able to fire at the vulnerable side or rear armor of something, and even if it missed half the time and had only one shot that was often enough to nudge them over towards being something your enemy would shoot at.
Counters and Weaknesses
The tactic wasn't really all that strong in the first place, just really good at playing off player mindsets, which is why it never really saw a need for nerfing; it had a tendency to either work really well or not at all, with no middle ground. The tactic flat-out would not work against MEQs unless the dice decided to bless you especially well, and the tactic had some difficulty against the Tau and Necrons. If everything went according to plan, the Angry Sentinel would either draw inordinate fire for a turn or two (in which case it did its job) or get stuck in for the Heavy Flamer shuffle against a vulnerable unit (same). The reason the tactic didn't work on Marines was that Sentinels' melee has no armor piercing ability and the Heavy Flamer was only AP4, though it could still do some nasty stuff to Scouts or Cultists.
The biggest weakness the tactic had was Sentinels themselves. Sentinels had only 10 armor and were open-topped, so they could (and did) have the ability to be punched to death by any unit that was S4 or higher. Ork units were especially prone to this, especially if the unit survived the initial Flamer+Assault. Their small squad size (often a solo Sentinel or pair) meant that they were easily outnumbered in assaults, and 10 all-around armor plus open-topped made it really easy to bring down Sentinels that overextended or simply got very unlucky.
As for counters, they were AV10 vehicles with open-topped. Start chucking Bolter or Shoota fire and one was likely to go down if they were exposed to enough Dakka, let alone anything with actual punch, like Autocannons or Zzap Gunz. This was why it was so important to march them using cover. The upside is that every single heavy weapon shot taken at your cheap-as-dirt Sentinel(s) was a shot not taken at your other units.
Evolution and Decline
The 5th Edition rules update wasn't the death of the tactic; the update nixed Regimental Doctrines, but also gave more tactical options to Sentinels in the process. Scout Sentinels were even better at closing the gaps, due to Move Through Cover and Fleet, and cost only 40 points each, but due to the loss of their doctrine rules, became more vulnerable in close-combat. An alternate option was to use Armored Sentinels instead, which, if armed the same way, clocked in at 60 points, and were both notably tougher and lacked Open-Topped. Scout Sentinels with Hunter-Killer Missiles still cost only 50 Points - a reduction from the previous edition, which helped offset the Doctrine loss. Parts of the Angry Sentinel tactic still work to this day, though 6th Edition and 7th Edition were both rather unkind to it.
What led to the tactic's decline was that, as of 5th Edition, changes to the Guard's Hellhound Tank made the unit a more appealing choice of damnations, for quite a few reasons.
The Hellhound was pretty much always the Angry Sentinel's main competitor for chasing enemies around with a Flamethrower, since the Hellhound was always a bit tougher and a bit more threatening. However, while the Hellhound could do about the same damage as two Sentinels with Flamers, and was much less likely to die if looked at, it cost almost 20% more. The Hellhound cost even more in 5th, which one might suppose would meant it was even worse, but in reality, the reverse was true; the Hellhound now boasted two variant versions that could threaten Marines (the Bane Wolf) and even Terminators or vehicles (the Devil Dog). Suddenly, Hellhounds weren't an opponent that MEQs could simply laugh off, and the things would subsequently draw fire like crazy, were tougher, and if, Emperor help them, the enemy did choose to ignore them, the price for doing so would be that much higher.