I'm so glad this isn't just me. I constantly do this and I'm so socially awkward I avoid going out. Some of my best are:
Visiting mum's friend who had just had a baby, I was 16. We stood cooing over the little baby and I came out with 'aw the poor little thing!' WTF. Clearly not what a new mum wants to hear. I still can't live it down and worst is that although mum and her sort of lost contact over the years she now works right near me, said baby is now about 18 and I can't bring myself to ask after him.
Suggestions for a work night out - 'I don't want to go there, it's full of lesbians' FFS, I have no idea why it would have been a problem even if was. This was to my colleagues, amongst whom there are gay men and lesbian women. They hated me after that, quite rightly so because I sounded like a twat.
Same colleagues - one of them had a car which was his pride and joy, he used to talk about it all the time and I just used to smile and nod whilst being bored senseless. There was an incident where a car of the same make and colour was vandalised, it was spray painted. Colleague asked me if I'd heard about it, my response 'yes, I thought it was yours and couldn't stop laughing'. He looked like I'd actually vandalised the car myself in the hopes it was his.
Job interview for a teaching assistant post - 'what would you do if a child had been rude to a member of staff?' Me: 'I'd force them to apologise, they're not getting away with that!' Wrong answer.
I also asked my bald uncle if he used shampoo or whether he was able to save money by not buying any.
Acquaintance I hadn't invited to my wedding: 'have a fabulous day on the 29th' me: 'oh I will, you too!' Yeah, that sounded like I was rubbing it in that she wasn't invited, made more embarrassing by the fact she really didn't care that she wasn't invited.
There are many, many more unfortunately.