My very first appliance change was similar to your story. I use the lock-n-roll two-piece and neglected to lock-n-roll prior to snapping my pouch on. I went about picking up my supplies since I thought I was done and felt the nasty outcome of this oversight right on top of my right foot. 
Not a good feeling at the time but funny now.
I have two other situations that I now laugh at. The first was my lesson with gas. A few days after that bag change I was sitting in the living room knapping in my recliner. Apparently I had eaten something that was a serious gas producer and as ileostomates know filters are for the most part are ineffective past day two (the main reason I use a two piece, ease of expelling built up gas). Well when I woke up and decided to get out of the chair (this chair does not return to the upright position gracefully) I heard a load "pop". It took a second to identify the source but when I realized it was the pouch I was mortified. 
Since I was completely dressed, undershirt (A shirt) and an outer shirt I ran to the bathroom holding my stomach. Once in there I saw the extent of the damage (looked like a poop bomb exploded on my belly) I was glad to see that it didn't cause the flange to dislodge just the pouch separated from the flange (two piece) and a wipe down, reconnect the pouch and change of clothes rectified the situation. Since I had UC, prior to the ostomy I had messed my pants many times but that was the first time I ever pooped my shirt, LOL. The phrase "never trust a fart" that UCers hold near and dear still holds true even after the surgery, LOL.
My second story just recently happened. I got the flu last week, I knew it was coming, the rest of the family had it prior. With me I have always gotten more of the bowel problems with the flu and seldom have the stomach problems. This time was no exception. Had I still had my plumbing I guess I probably would have been woke up with an incredible urgency to go. Without my plumbing this allowed me to sleep peacefully right through the start. At around 6 a.m. I felt a cold wet feeling on my belly. I assumed it was one of the two lap dogs we have burrowing under the covers and sticking their nose on my belly. I was just getting ready to remove this little inconvenience when I felt a now warm wet feeling. Light on, covers back revealed my pouch, completely full, laying next me in bed. Nothing had spilled out but it was full. I got up rather quickly (that caused a spill or two) grabbing the pouch and heading for the restroom which the quick movement obviously spurred my system into overdrive. Leo was spouting off at his best making me look like one of those statue fountains you see in parks and such. I made to the restroom and was perplexed trying to figure out how to open the tail of the pouch to empty it without it being attached (one hand holding it and only having one hand to undue to lock-n-roll) all the while Leo is redecorating the wall behind the toilet. Finally common sense took hold 
and yes I just dumped it out through to top hole, 
, rinsed it real quick and put it back on stopping the early morning redecoration. This caused me and my wife alot of clean up but after the damage was done all we could do was laugh. I found the pouch was convenient for having the flu (no urgency issues) but staying awake was the key. That night because of all of this I set my alarm for every hour so I could wake up to make sure I wasn't still filling my pouch at the alarming rate it did during the day.
John