One so old it has pretty much switched meanings...
'blood is thicker than water' - meaning family is more important than friendship.
In fact the original phrase was:
'the blood of the covenant is thicker than the water of the womb'.
Meaning the bonds you choose to make are stronger than those you had no choice in.
Another...
'Great minds think alike' - ie, we all agree therefore we're probably right/clever, however the original saying was:
Great minds think alike, although fools seldom differ' - just because we all agree, doesn't necessarily mean we're right/clever/smart...
The one that REALLY drives me up the bloody wall though is the Americanism...
'I could care less' - being used to mean the opposite 'I could NOT care less' which is what the phrase should mean. Theres no logical explaination for it, its just bloody stupid.