How about mistakes in common idiomatic expressions:
'To tow the line' or 'To toe the line'
'A shoe in' or 'A shoo in'
'Font of knowledge' or 'fount of knowledge'
Then there are mistakes that are just plain funny. I received a letter from the council once. Instead of the final line 'Sorry for any inconvenience caused' it read 'Sorry for any incontinence caused'. Needless to say, I pissed myself laughing...
