Poor change testing
Going in to a change with no certainty (or even a reasonable prospect) that it will work. Whether that's because of lack of time to work out how to do the change and then carry it out, live - or a lack of preparation due to test/mirror systems that don't actually reflect the production environment - changes being assigned to staff who have no real clue what they're doing (but it was their turn) - or even just systems that are intrinsically untestable and have to be changed "hot". One place I was involved with made over 12,000 production changes a year. Of those 60% were to undo earlier changes, correct improperly applied changes or to add diagnostics so the support teams could work out what was going on - oh yes: then another change to fix the problem and another change after that to take the diagnostics off again.