This one is pretty bad for cringe-factor:
I have a happy, playful relationship with my girlfriend - when she walks by, I will pinch her, poke her, tickle her, etc. We've been dating 6 years, so most of these things are ingrained into my muscle memory at this point, and I do them without thinking. She'll usually giggle and go about her business.
There was a coworker at one of my old jobs that looked a little bit like my girlfriend. I wasn't attracted to her or anything, but I guess sometimes my subconscious was tricked. You probably see where this is going.
One day she bent over to pick something up - I swear to god I wasn't trying to be creepy or sexual, I just did it without even thinking - I slapped her on the butt. The split second after my hand made contact, my vision blurred, my blood pressure spiked, and I screamed "GAAAHHHHH WHAT THE FUCK WAS THAT."
She turned around red-faced and I kept screaming words that didn't form sentences. It was the most horrendous, awkward moment ever. We were good enough friends that she eventually laughed about it (the office teased me about it for months), but that was the worst 30 seconds of my life.