There were at least two giant ones that stick out in my mind.
First, there was the Taboo on Voldemort's name. Ron just conveniently starts to insist they not use i? Would it have been so hard just to have them hear about the Taboo while in the Ministry? That way it wouldn't be such a blatant plot device.
Second, there's Harry not realizing he had to use parseltongue to open the Locket until just when it was convenient for the plot.
There's probably more. Does anyone else have examples?