Stem leaks are not uncommon and may only be conspicuous under pressure.
I see them more on older tubes, but certainly anything can fail anytime.
cracks in Schraeders, cuts, and seals to metal in both Schraeders and Prestas.
Can be spontaneous.
My last one was while I was sitting drinking coffee next to the bike.
Makes riding with a spare tube or phone more reasonable, usually only happens when I left them behind.