Often, it will go bang, but sometimes, nothing obvious will happen.
I was refurbishing a Sony BVM 2000 Monitor one time, & found a number of electrolytics on one board were installed "the right way round" according to the "silk screened" markings on the component side of the PCB, but reversed, according to the markings on the other side, as well as the schematic.
This device had been in service, meeting all specifications, for around 10 years!
Needless to say, I replaced them the right way around as per the schematic.
On another occasion, "in another place", I replaced all the electros (big ones) in a rack mounted PSU in an AM Broadcast Tx site "speech input room".
Unfortunately, it was a -24v supply, & I mindlessly fitted the caps as for a positive supply.
At turn on, there was a loud "Bang!" & the air was full of capacitor guts----it looked like it was snowing!
On another occasion, a trainee gave me his project (A Dick Smith Electronics kit), to check the wiring.
From a cursory inspection, all was well, right components, excellent soldering, & so on.
Having got my approval, he switched it on.
There was a sound like a 0.22 rifle shot, & the case of an electrolytic blew its end open (It was a small cap-- the big ones sounded like a shotgun)
Dear old DSE had marked the board wrongly, so I wouldn't have been able to pick it without a much closer look.
Even so, I think the trainee lost a bit of faith in my supervisory capacities!