What Would Happen to Retainers Accidentally Washed in Hot Water?

Snap them in and see if they fit like before. Usually, they have to be boiled before they distort and the water in a washing machine isn't hot enough to distort them.

I guess it depends on how hot the water is. I wash mine in hot tap water all the time - but not a dishwasher or with boiling water. If the aligners become distorted, you will know when you try to get them back on. If they are distorted, call your dentist and pay to have more of the same number of aligners shipped to you. In the mean time, step back in the one-past set of aligners to hold your teeth while you wait for the shipment to arrive.

If it FEELS like there was no change (after putting it back on) then there likely wasn't any distortion. Brief (less than a second) exposures likely aren't a big deal. Boiling them for a minute to "sterilize" them are a very bad idea... They WOULD distort and need to be remade.