When Sir William is called to the Bruce's place, he is about to go and he is talking to his friend Hayebish and when William answers Hayebish he says "look at this" in his american voice and then changes to scottish.
Actually, Mel does that a few times during the movie and he also overdoes the Scottish accent in a few places. IMHO, I don't consider that a slip up. He does a far better job with his accent than Brad Pitt did in "The Devil's Own". Now that's a hoot and a half :) Granted, Pitt was consistant -- consistantly awful.
Btw, I don't remember Gibson having a cold in the movie, so who the hiddley is "Hayebish"? The name is Hamish.
I agree - Mel's Scottish accent was fairly good and superior to most attempts at a Scottish accent. The ususal Scottish accent we hear from non-scottish actors is usually the Scottish/Irish 'one size fits all' accent.
Well to my ears, and i may be wrong, some scottish words sound a bit american... it may not to a scotsman or to an american but yeah we aussies have a bit of a mixed up accent... :D
Considering the fact that the characters wouldn't have been speaking with a Scottish accent like we are familiar with today (indeed, they wouldn't have even been speaking English at that time) I don't think this should be considered a slip up.