I'd like a penny for every time I have passed traffic information to 'VFR' and 'SVFR' flights (yes...SVFR too...) only to be told 'Roger, I'm IMC at the moment...'
Ah, but you might be just in a bit of muck for a couple of seconds.
Happened to me on the way to the Sherburn meet in March, I was crossing Doncaster zone on a VFR Traffic service no higher than 2,500'. It was good vis but there was a lot of cloud around at varying levels. The good lady controller passed me some crossing traffic just as I popped into some scud (you couldn't really fly around it, you would just fly into some more and it went down quite a bit), I called 'IMC this instant, watching on exit' which was literally one or two seconds later.
She called ' Can I remind you that you are on a VFR transit.' I said it would be no problem to go IFR and asked for an IFR clearance. She said 'No, that's OK stay VFR.'
I was left a little bemused. I might add that Doncaster controllers are always excellent.