This is how I sometimes
refer to something that has gone
wrong or turned out slightly
unusual. I might say "That attempt to assemble the flat-pack chest of drawers went a bit
pear shaped".
I also have a tendency to refer to nights out this way. A pear shaped night could start off with a quiet drink in the pub. Getting invited to a gig, ending up walking home barefoot at 3am, not having eaten since 12:30pm. (Yes, this has actually happened to me)
Where does it come from? Well, imagine a perfect circle. Then put a pear shaped circle next to it. I think the comparison speaks for itself.