This one is due to Marcus de Sautoy but it came up last night again.
It's the ultimate insult to a mathematician, the one that makes you reach for your gun.
Don't say "it's not rigorous".
If you say "It's not elegant" then we can argue or we can admit "yeah it's not really, maybe we can find a neater way". (Or just "heigh-ho".)
If you say "It's not clear" then we reply "your problem, not mine". (When a mathematician says "clearly .." they mean "think about it a bit and you'll see". When it's clear they say "trivially".)
If you say "It's not useful", we say "so what, who cares?"
If you say "It's not true" then we say "That's life and the universe, deal with it".
If you say "It's not rigorous" then you better be able to back up that statement, you bet. Or we are gonna fall out.