Big Issue sellers that ask you three times in the space of 5 minutes if you want to buy their "last" copy.
Not that I have anything against BI sellers, in fact I think it is great that the majority are selling it to get their feet back on the ground, and I once spent an interesting afternoon chatting to one in Leeds, but please please please, if I said no the first time you asked I am not going to change my mind just because you asked again.
Courgettes
Aubergines (they both should be tried at The Hague for crimes against decent food kind)