I have never skip-lagged once on UA, and people seem to think doing it once is no big deal, but I am wondering what would happen if I skipped XXX on SIN-SFO-XXX and instead jumped on SFO-YYY on a separate ticket (changing to SIN-SFO-YYY doesn't work due to the timing, not cost, of the flight(s) being offered--in particular, UA is only offering SIN-SFO-YYY on the evening SIN-SFO flight and I need to take the morning one).
I know UA flags double booking on the same flight; not sure if I'd be flagged here with separate flights from SFO that don't actually overlap on timing.