If you happen to do it in an Aztec, like I have, you will lose all the fuel from that tank....and very quickly!
Another gotcha which I have seen twice is leaving the keys in the cabin door, and then forgetting about them. Completely fcuked the day of a Lear 45 pilot when the keys came out soon after take off and went straight through the port engine. Same thing happened to a friend, but he was luckier - the keys bounced off the nacelle and just got lost somewhere over the Sussex countryside.