Rusty power supplies suck. I never dare to turn on a computer that even shows a little bit of rust near the PSU.

and rusty floppy drives, you'll just get your disk stuck in there and it won't work anyway.

I've rescued computers and CRT TVs/monitors that have been left out in the rain, I'd say that rust is the main enemy. Edge connectors and component legs will rust if not dried soon enough, causing intermittent connections, short circuits or even breakage (when component legs are so brittle that they break off just by touching them I consider it not worth repairing), but most things survive just fine if left to dry as soon as possible.

Where I live the rain is dirty because of the high air pollution and thus leaves a very recognizable layer/pattern of filth on cases and circuit boards, making it evident to the trained eye when something has been rained on. Anything you can do to speed up the drying process (disassembly, heat, air circulation, etc) will be beneficial. Non-sealed mechanical components will need special attention, and so do high voltage electronics like CRTs - if water gets into the tube socket it can cause bad focus (usually accompanied by the sound of frying bacon) and internal arcing leading to flashes, shutdowns or blown components. The tube socket is difficult to dry naturally, I usually spray a bit of contact cleaner on the pin holes to displace any trapped water followed by application of a hair dryer. If arcing continues, the socket could be internally rusted and might need to be replaced.

I've rescued a few PC's out of the rain over the years - dried 'em out and they worked fine. PSU's are the concern though so yes do make sure that it's completely dry and rust free.

Yeah I would just leave them to dry for a few days before testing them out, but yeah I've rescued machines from the snow and rain before. 🤣

I even washed a computer once, was a dirty smokers A2000. Still working fine.

But be sure its dry, totally. A little water does not harm when the is no power on it. The main problem is rust and corrosion. Most concern is the power supply and the backside of the mainboard and cards. The most stuff inside won't get any wet in a normal rain.

This is also a reason that I have a separate fuse in my hobby room for some outlets near my working space. 😁

I rescued a 386sx that was one of the many things supporting a bus(!) which had a missing tire. It spent a whole winter under a snow, but did function except the HDD which had died. 😢

My first computer was gotten after a tornado hit where my dad was working. Computers thrown all over the place and soaking wet. He got a few and brought them home, then for the next 2 weeks they got air dried

I dont remember if I've ever had PCs that were out in the rain, but ive had televisions that have been. let them dry over a few days, and theyre good.