What kind of flat is this?
I am a very modest bike mechanic. I can change or patch a tube, set derailleur limiters, and I think I tried to true a wheel without ruining it once.
My wife's bike had a flat after a ride this week. The whole ride we didn't notice, but the next day it was entirely flat. I pumped it up and could hear and feel air escaping next to the valve stem (not out of the stem, it continued even when the stem was covered/plugged). I replaced the tube, no big deal. I assumed that the tube had been compromised around the valve stem hole in the rim or pinched, etc.
I inflated the tube after I had swapped it out, thinking I could patch it to be a spare again... and it just holds air now. It has no leaks. What the...