I'm working on compiling a list of "gotcha" bits from the Tau codex... those easily-overlooked rules bits that could end up screwing you over when you discover that you've been playing the army incorrectly at the most inopportune moments. Let's start, shall we?
- If taken in an allied detachment, Farsight can still have his 7 Bodyguards, but he doesn't deep strike without scattering. That's a Warlord Trait, which he doesn't get to use if he's not the Warlord (which he can't be because he's not in the primary detachment).
- Multi-trackers only work in the Shooting Phase. In Overwatch, you're still limited to your standard number of weapons you can fire (2 for Riptides, 1 for everyone else - and that includes Shadowsun).
- Longstrike's Hammerhead is not limited to Str 5 weapons or less when Overwatching, because he doesn't have a Point Defense System. His armor just grants his tank unlimited Overwatch.
- Because they aren't Flyers, Sky Rays aren't limited to the 2-Missiles-Per-Turn firing limit. It's just a vehicle with 6 one-shot weapons. Note that on the actual Flyers, the Seekers are counted towards the limit.
- Beware taking drones on lone suits, like a Riptide. Losing one drone will be enough to force a morale check, which you may just fail.
- The extra shot from a Cadre Fireblade's Volley Fire only applies in the Shooting Phase; you do not gain the extra shot in Overwatch. However, an Ethereal's Storm of Fire buff does work in Overwatch.
- Using Markerlight hits while firing Snap Shots to raise a unit's Ballistics Skill does not make the shot no longer a Snap Shot; it just makes the attack a Snap Shot at a higher BS. That means that the unit still can't fire Blast weapons, such as overcharged ion guns or railgun submunitions. This also applies to the Counterfire Defense System.
- Signature Systems are one-per-army, not one per detachment. You don't get to double up on them at 2000+ points. Airbursting Fragmentation Projectors and Cyclic Ion Blasters, on the other hand, are one per detachment.
That's all I have for now. Keep in mind, any and all of these could be FAQ'd to irrelevancy at any time. If you've found a Tau "gotcha", comment below and let me know!