Bad things:
Not teaching students how (and why you should) lean the mixture.
Not fixing anything that goes INOP unless the failure actually grounds the aircraft.
Telling students never to prime with the throttle, without telling them WHY (fire risk aside) it appears to be a more effective way of starting a cold engine. Some aircraft actually prescribe it in the POH.
Giving students an aircraft for their skills test that they've never flown before and which has a very different instrument and switches layout from everything else they've flown.
Not telling students that they ought to be carrying a copy of Pooleys on their skills test because the examiner may well divert them to an unfamiliar airfield, ask them to dig out the plate and fly some circuits.
Letting the day's schedule slip such that a student who has moved heaven and earth at work to get to the airfield for 1800 arrives to find their instructor still in the air. If an earlier student was late, then their lesson should still finish on time.
Not specifically teaching it, but giving students the impression that you should fly everywhere at 3,000ft.
Teaching IMC students to intercept the localiser and set full flap and 65 knots (PA28) before descending on the glideslope, thus allowing the grass to grow so long it obscures the runway before decision height is reached. Also once you finally land, the annual is due and you can't take off again to go home.
Teaching students to clog up a busy LARS frequency on the weekend by asking for a basic service (slowly, with lots of umms, ahhs and errs and unnecessary info like the number of people on board) when going for a local bimble.
Not explaining to students that the likelihood or otherwise of getting class D transitions and SVFR clearances is largely dependent on how slick your RT is, because that's how the controller forms an opinion about whether you're capable enough to be allowed in their airspace.
Not showing students what it's like to receive a traffic or deconfliction service during their training, so they're scared to ask for one once they have their license because they don't know what will happen.
Hammering the 'maintain VMC at all costs' message to such an extent that new PPLs confronted with a combination of cloud and mountains cannot bring themselves to climb, climb and climb some more. Even if I wasn't instrument-qualified, I'd rather fly into a cloud than a mountain.
Teaching students that the IMCr is an 'emergency get you home rating'. It isn't. It's an instrument qualification that allows you to fly in IMC (OCAS and in class D) and make instrument approaches as a matter of routine. If you don't use the skills with some degree of regularity, then they'll be rusty when you need them to 'get you home'.