I started therapy a few weeks before my parents went on a several week vacation. My therapist encouraged me to use the freedom to experiment with my presentation, so I did. The day they flew back I did all my laundry, my girl clothes first to get that out of the way, then my work clothes. I apparently had a pair of panties in with my work stuff that I didn't notice. My mom started her laundry that night and I had left my work clothes in the dryer, thinking I had time in the morning to take care of it. So when I went to take care of it in the morning I found a neatly folded pile of clothes, including my panties in the middle. I just about died right there...but she never brought it up.
When I finally came out to her she said she had rationalized that in some way and didn't think too much about it, while I had a massive panic attack

edit:
Thinking about it more - I used to keep a diary/journal type thing on a PDA (back before phones did everything under the sun). I lost the PDA on a city bus one day, that freaked me out for a while. I got over it though because I didn't actually have anything identifiable on it - just some notes and the journal. So somewhere out there is someone who found a PDA with a whole lot of messages about how very badly I wished I had been a girl back in like 1999. If only things had clicked into place back then!
I haven't had any other "close calls" though, as far as I know. And definitely not with technology, after losing the PDA I got super paranoid about protecting data. I'll admit I take way more risks of people finding things now because I have the mindset that it's just a matter of time anyway. Plus it would kick my ass into coming out to more people...maybe my subconscious is forcing the issue