My worst habit is accidentally leaving myself logged into FH while I'm off working or busying around alt-tab. I've often came back to missed whispers-- which has admittedly lead to a bit of 'why are you ignoring me' friction between my friends. Luckily, most tend to quickly catch onto my vanishing act!
Otherwise, leaving groups and constant group-hopping is another awful habit. It's very rare for me to feel comfortable and welcome within one-- they tend to get a bit overwhelming. I desperately wish I could mesh with them!