Technically not the worst (she was actually very enjoyable), but up there with the worst experiences. I'd come up to Liverpool one Christmas from London and ended up going straight to some social club where me ma's works do was on. Didn't care, knew a few, so thought I'd get into the ale and see what transpired. I was in my mid-late 20s, and spotted a chick who I figured was 2-3 years older - petite, quite a stunner in her own way. Got chatting to her, boogie on the dancefloor, and end up leaving my bag with my ma to take back to her place, as "I'd pulled". (She was an understanding sort).
Anyway get back to this lass' place and it all starts to unravel. Turns out she thought I was mid-late 30s, and she was late 40s (divorced, no kids, sterilised <--- she felt she had to tell me that bit). Worse still, when she found out who my mother was that was an absolute no no. By this stage the ale had worn off, and she still looked fit, and no way her age, so I'm practically begging her to reconsider. In the end after a few coffees, I said I was leaving and to have a good Xmas and all that.. and as I walked to the door, she, despite her tiny frame, dragged me up the stairs into her bedroom.
This would've been fine, except she had this really irritating yappy little terrier that obviously didn't like me. And she refused to shut the little mutt out and insisted the bedroom door wasn't closed in case it upset him. In full horn mode I'd have agreed to just about anything and so was banging away quite merrily, boring missionary at first but thinking the next few hours were gonna see if she was as pliable as she was ploughable, when the hairy rat obviously mistook her cries for me attacking her and burst into the bedroom, leapt onto the bed and sank it's teeth right into my arse.
Suffice to say that was it, all over just as we'd got started. I swiped the little get off the bed, she got upset and I didn't need telling twice to go. Walk of shame at 6am with a bit of a limp.