Deevah .. don't say you didn't ask ...
The zip fracturing disaster has happened to me at least in these circumstances: ... in the street, on a bike (more to be expected, maybe), in a Chinese restaurant, at a lecture ...
I have also (very occasionally) been responsible for destroying the fabric or a seam rather than the zip.
This was more a problem when I was a lot younger than it is now, but it can happen even now. I don't tend to find a problem with formal clothes like suits which are tailored or altered to fit, but with casual off the peg trousers. When I was a student the fashion for cords did me no favours at all (that dates me, I know!)
Somtimes an erection would cause a pretty loud rip and I knew immediately what had happened, at other times I would only gradually realise I was feeling cooler and less cramped than usual ... and look down to see my underwear bulging out of a gaping hole ... Talk about a cold feeling at the base of the spine!
At the moment I own two pairs of trousers where the zip has gone ... in one case I discovered this after getting home from a party, not at all a good feeling.
One story with the detail you asked for. This happened at univ during a lecture ... no special reason, just the way I was sitting maybe and a feeling of well-being, up he comes ... rip. So I cycled home to my "digs" as fast and discreetly as I could, to change. I shared the digs with a close friend of mine (still my closest friend). It was mid-morning. While searching for a new pair I took Mr Happy out of my jock to give him some room and some air ... he was at least semi erect, and gaining ground fast ... when in walks my 65 year old landlady without knocking, not expecting me to be there at that time, to clean the room.
She stared (rabbit at headlights style) without saying anything for what seemed like a full minute, and I was too embarrassed to speak. Eventually she apologised, and I explained what had happened and why I was there.
She said that she could see the cause of the problem. I had never until that time ranked very high for her ... my flat mate was a very handsome young man and she had a major soft spot for him ... but from that day on I could never put a foot wrong with her, and she couldn't do enough for me.
She went right out that day and bought heavy duty zips for all my trousers, and sewed them on for me. She said it was a pleasure to do it for me and wouldn't take payment ... At least "he" won me an admirer, even if in pretty bizarre circumstances.
Sorry, but you did ask.
Max