The easier question is what haven't I seen
Some horrors include:-
- pairs too far untwisted
- cables kinked or otherwise bent radius exceeded
- different standards used for punching at each end A Vs B
- no basic continuity tests done to check all pins are actually connected.
- individual jackets unnecessarily stripped before punching down
- wires poked into without using punch down tool
- cables cut too short to punch down and then extended using jelly crimps or chocolate block connectors (because it's fine it's not mains voltage!)
- mismatched faceplates e.g. cat6 used on cat5e cable
