An over use of the words 'hmm' 'urmm' 'ummm' 'Mhmm' or any other derivative. It's when people are thinking too much but have a lack of anything better to say.
Me: He left me, so rude, I wonder if I have done something wrong?
Me: he says I've changed, do you think I have?
Matt: hmmm.. oh no, I double hmm'd. =O
Me: Oh noes, you've got Hmm disease!!