A few that come to mind are:
- sellers that mess with the scans to try and create a wrong impression of the card - this comes in a few forms - trying to obscure issues by providing bad scans or doctoring the scans to make the cards look sharp when they are not.
- auction houses only giving one of the two pop reports on a card and claiming it is the highest graded by PSA when SGC has graded 4 higher.