Fish of Fury
This is why people hated Tau players with a burning hatred rivalling the rage Angry Marines have for Cultist-chan. Every single bad joke made about Tau players forevermore is pretty much entirely justified due to how annoying, stupid, and prevalent this was until the ruleset and codex updates.
Even veteran Tau players openly criticized this bullshit after it swept tournament after tournament after tournament. Most players that aren't retarded are glad this shit is gone, and cheerfully tell those who used this trick to enjoy their AIDS.
How it Worked
Previously, the Devilfish transport for the Tau counted as a skimmer at all times. It was also very large for a transport (a Devilfish is notably longer than a Chimera, for example). The general gist is that a player would park 2 Devilfish close together in a chevron formation, have the troops disembark, and fire from a basically assault-proof bunker caused by the pair of Devilfish. Units couldn't charge around the 'Fish (because they were too big and due to the rules on assaults), and while they could assault the 'Fish itself, Skimmer rules only let you hit on 6's, so it was likely to be a futile gesture.
The tactic was insanely powerful, though not unbeatable - canny players knew the disadvantages Devilfish had and would ergo target them with lots of heavy weaponry - but because of Disruption Pods, Devilfish often proved surprisingly difficult for many factions - especially those with limited anti-tank options, such as the Orks, Tyranids, and Necrons. The 5th-ed tactic of rushing in a METAL BOX and unloading Meltaguns into the fish was suicidal, because 4th ed brought us the chance to destroy vehicles with glancing hits - and from 2 full formations of Fire Warriors (not to mention the Devilfish transports if they survived) would mean a mulched Rhino and the Marines inside facing down 30+ S5 shots in rapid fire range.
The only reliable tricks to beat Fish of Fury were deployed by Chaos, the Imperial Guard, the Inquisition, and the Space Marines. These four factions either had ways to get big ugly close-combat troops right into the enemy's midst (Chaos, Inquisition, Space Marines), Artillery which could fuck up the shit of the entire Devilfish-Bunker (Imperial Guard, Inquisition, Space Marines), or big fucking ordnance weapons that could render the tactic a moot point (all of the above). Even for these factions, however, the tactic was infuriatingly hard to stop; one Devilfish-bunker could dictate the flow of an entire battle, and attempts to destroy the damned bunker would invariably be met by Stealthsuit and Crisis Suit teams ambushing your forces, markerlights showering everything, and of course, railguns.
This naturally led to thread after thread of "HOW THE FUCK DO I STOP FISH OF FURY" and "FUCKING NERF MECH TAU" threads on every Games Workshop official forum (and, of course, /tg/). The bullshit saw no signs of abatement and saw use with just about every Tau army in the official tournaments. The hate for it was not limited to non-Tau players, either: It was so prevalent and so hated that large numbers of Tau players went to increasingly-absurd lengths just to avoid any association whatsoever with it; players would openly footslog Fire Warriors just so they would not be associated with Devilfish-induced cheese.
The tactic became a self-sustaining nuclear fireball of fail and derp. Like several equally-broken things before it, this thing needed to be fixed before it managed to warp the metagame even further - and in a surprisingly uncharacteristic maneuver for Games Workshop, they updated the Tau codex, which was easily the youngest of the set. This update, and the one succeeding it, were what finally led to the tactic's demise.
The bullshit finally stopped with the new ruleset and how it covered line-of-sight, paired with the fact that the Devilfish now automatically lands if it's immobile for a turn, losing skimmer status (the latter rule has since been reverted as it became unnecessary). Then 5E rules said skimmers were treated just like other vehicles in regards to close combat (so not universally hit on 6's) The new armor rules (glancing hits can't destroy a vehicle) also helped. A textbook example of how occasionally Games Workshop gets its head in the game and stops sucking for a few minutes, it's now possible to blow the fucking things up with Meltabombs and the onslaught of RAGE brought about by this tactic is now a thing of the past, making the tactic only about as effective as Mechanized Guard Chimera attacks (which was frankly how it fucking should have always been).
...Of course, the TLOS in 5th Edition means you can now do this with Rhinos and smoke instead, so it's apparently OK for the Space Marines to do it. THE ENEMY HIDES BEHIND METAL BAWKSES!! THE COWARDS! THE FEWLES!!!
...To be fair, however, Space Marines aren't able to rapid fire at S5 and the BS of however many markerlights you feel like using, which was a big part of the RAGE, so there's that.
Suddenly More Updates
Come Sixth Edition, transports have now been forced to forgo their full speed, in favour of their cargo now being able to move after climbing out, meaning that once again, the idea of Devilfish swooping in from nowhere and planting themselves between the enemy and their cargo has just taken another serious blow.
With Seventh, it's officially dead and gone as if it weren't before. You can do something called the Sea Turtle, where you disembark from a Devilfish, shoot, then move the Devilfish Flat Out to get back in front of the Fire Warriors. But, the Sea Turtle is only mildly infuriating, it sacrifices any shooting from the Devilfish's guns (which, while not as nasty as a Wave Serpent or Ghost Ark, are still decent), and you can assault skimmers, even when they move Flat Out. And then you have things like Dark Reapers and Dominions, who just say "fuck your jinking" and shoot the Devilfish to death anyway.
In 8th edition it's... usable and pretty decent. While you lost the old skimmer rules, Devilfish are enormously more tough this time around, with 2 more wounds than a Rhino or Chimera. You can move them and fire all their weapons in the same turn, plus you can now disembark your troops and move the 'Fish in front of them in a single move action. It can also charge at something to force it into melee so its cargo can't be attacked, then fall back on the next turn while still being able to shoot (due to having the FLY keyword). Most races have means of tearing through its armor and dealing multiple wounds, but with a little creativity it can still save your blue hide. Breacher squads are a particularly good fit for it, since having the Devilfish charge will let them get up close without risking getting caught in melee themselves.