I once went to a friends birthday meal at a curry house, as we were all students it was understood to be the type where you pay for your own food and drinks and chip in a few quid each towards the birthday girls meal. This is normally fine, but on this occasion a rather flaky friend (who I hadn't met before) showed up about an hour or so late. She had missed the ordering part of the evening but arrived just in time for the eating. She managed to get several spoonfulls of everyone's different curries and side dishes with a bit of rice, some spare naan and even a few glasses of wine from the bottles on the table! It was enough to be a full meal but when we were splitting the bill she didn't put anything in as she hadn't actually ordered anything.
This must have been about 4 or 5 years ago but I still think it was shoddy behaviour.
In response to the OP I would always expect to pay for my own meal regardless of who invited me or what the occasion was. I guess it's just something that varies with geography/age/income bracket or some other factor.