Based on the design, if the spring's in place and the pin's inserted correctly it can't fall out. If the spring's not in place or the pin's not fully inserted it will fall out. This is a common problem with Kel-tec PF-9's which use a similar assembly pin. Unfortunately, it's easy to push the pin down and against the spring (rather than over it). This pushes the spring out of place so nothing's retaining the pin. The first indication of trouble would be the slide falling off.

My plan it to check the spring position every time I take the gun down...then be sure I push the pin in over the spring when I reinsert it. Once it's together I'll check to be sure the pin is flush against the frame and press down (slightly) on the lever a few times to be sure there's spring resistance pushing it back up. If there is, I'll know the spring's in place and the pin should stay in. Nothing's for certain of course...

Milspec