For myself it's getting blush right - I've only taken to wearing it recently and it's a bit embarrassing when you go heavy-handed with the stuff
On the other hand I've found a few neat tricks to reverse the damage if necessary!
On others: Mascara - I've only so far done makeup for brides, bridesmaids and mothers - people who are normally not used to having their makeup done and it's made worse when they start blinking all over the place (please just rest your eyes I may do more damage if you blink than if you don't)!
And the other is matching foundation, with the first few clients I had I made them too pale on camera (although they seemed fine in the flesh). It's a nightmare and it's something I keep practicing on and won't stop until I find the cure