I almost never crack, but I remember this one time a few years ago. My sister had just had her third baby, so I was babysitting the first two kids while she recovered. Anyway, this was during my winter break from university, so I was really looking forward to some good quality time by myself. Instead, I had to babysit two very active kids all day. My sister didn't want the kids watching more than an hour of tv a day (that's what they do at their house usually). I was babysitting at my parent's house, though, where there were hardly any toys for the kids to play with, so it was pretty much me entertaining them all day. They'd wake up super early everyday, too!
Anyway, my neice was going through an "I hate everyone but Grandma and Mommy" phase, and this one day she wouldn't stop crying all day and kept hiccuping about how she didn't like me. I was sleep-deprived, I had absolutely no "me" time in the last week, this was supposed to be my vacation, and I was obviously not doing a good job with the kids anyway. I snapped. When my mom came home from work, I got really angry and yelled about how I never wanted kids of my own and stormed off (note: I usually love kids and I'm the "favorite aunt" because I bribe kids with kisses, presents, and candy). I totally lost my cool. Later, I started sobbing into my mom's arms for, like, half an hour. I NEVER ever cry! It was surreal.
I chalk the whole thing up to sleep deprivation and not having enough introverted time. I felt like everything was out of control, that I wasn't even making a difference, and that I was suffering needlessly. I think that's probably the worst breakdown I've ever had in my whole life. When my SFJ sisters lived with me, they'd breakdown like that about twice a week...