Buying a boat: Tell me you horror stories

In moment of madness, we are thinking about taking the plunge and buying a boat - Nothing too fancy. Something, small, old but seaworthy that we can take out when we want to but not feel guilty if she sits on the mooring for weeks on end. I would love any tips you have. I have scoured the web for checklists and found some useful ones but I am looking for voices of experience. Your best tips and your horror stories? Actually the more horror stories the better. In fact screw the tips, what's the worst experience you have had buying a boat? What should I avoid at all costs?