What happens on a lapse in SM-2?

A failed review ($q<3$) resets the repetition count and sends the interval straight back to 1 day.

The card re-enters the schedule as if newly learned: $I_1 = 1$ day again, and you climb the interval ladder from scratch. Notably, in classic SM-2 the ease factor is not reset — only the interval and repetition count are — so a card you keep failing keeps its (possibly already-lowered) EF.

This "back to day 1" reset is deliberately harsh: if you've forgotten it, you clearly need to see it soon.