> Without ECC RAM, how would you know that you had a single bit flip?
Errors, glitches, or files with unexplained errors. If there is a bit flip in code, more likely than not that'll be an invalid instruction, jump, or something like that, so it would crash. If it's in data, often there's some algorithm involved (compression, linked list, js/css/html, whatever), so that would either find invalid data and crash or display an error, or display at least a wrong pixel to a certain degree (but, true, I'd have to spot a single color channel being off and there's, let's say, a 4/8 chance of the significance being too low to really see; but this only applies to a raw bitmap case). Data on disk should also become corrupted (I'm thinking of image or video editing, data processing like gigabytes of port scans that I recently processed, etc.) if it was inadvertently modified in RAM. There are a ton of ways to notice this, though admittedly also a certain number of cases where one wouldn't.
I see your point though, like, if the error is silently corrected by the software or I just hit retry and don't figure what the error must have been (also because it won't be reproducible), I'm unlikely to find out that I need ECC. Maybe I should introduce random bit flips on a raspberry pi (so I don't corrupt my actual production filesystem) and use that for an evening for browsing / programming / other usual activities to either prove or disprove the theory that I'd notice if this happens with any sort of regularity.