1) when a snag starts appearing across a fleet (once the crew have talk to each other, during all their time off)
e.g. trim on one aircraft XXXX had a defects, next minute you know the week after on other aircraft YYYYY, ZZZZ you get defects like "trim feels strange, please test etc",
2) snagging a button when you know they have used excessive force on the FMC, GPS etc but don't admit it
3) pilots who have left/furlonged/made redunant/fired from major carries, join a regional outfit and still think they flying a major (defects such as crack on light cover..... yep its an interior cover and the crack is just visible behind the fastener and maybe in your previous airline while you where boarded and notice the crack, the taxpayer went off an bought you another cover at an inflated price from Boeing. But wait stupid expenses like that resulted in layoffs). Applies to cabin staff too with comments such as "When I flew for {insert national carrier name} we had expresso makers" well hold on while I get my credit card out and buy you one to fit (no need to worry about certification problems etc
)
4) First officers just after they made Captain, every noise and creak freaks them out.