From the student's point of view:
Taking the afternoon off work on a glorious day, arriving at the club to find your instructor 'has a migraine', when you know full well he was on the sauce until the wee hours the night before!
As the student is attempting to try a diversion on the map, performing S turns and then asking why the line isn't straight.
Moaning about pay (every lesson)
As the poor student is carefully maintaining height, airspeed and track, saying 'I have control', standing the a/c on it's wingtip and pointing out some disused airfield before handing control back to the student 500ft lower, at 85 knots and 10 degrees off-track. Then saying 'height!'
Contradicting everything the last instructor said.
Never buying a round as 'you're skint' but assuring the student you'll buy next time.
Continually giving the student a knackered old dog to fly, whilst telling him 'it's the pride of the fleet' and then sniggering at the studie's gullibility with your fellow instructors!
And the cardinal sin, inviting the student for a drink, getting them to pay and then disappearing off to the corner of the bar with your mates / fellow instructors / airfield employees, leaving the student to speak to Reginald, the idiot savant spotter with a personal hygiene problem. Then sniggering at the studie's gullibility!
BTW, no longer a student!
