Virtually never in practice (they are corrected) if you use ECC. A server that doesn't is weird. TBH any computer that doesn't is weird but the industry seems to consider it normal to have random computational unreliability because of that pretty much only unprotected component (Ram without ECC) in consumer hw.
> We also carry out a retrospective analysis of historical SSH scan data collected over the course of seven years, and find that these invalid signatures and vulnerable devices are surprisingly common over time.
> Our combined dataset of around 5.2 billion SSH records contained more than 590,000 invalid RSA signatures.
Seems like over long periods, it can occur a spoopy amount of time.