...when I'm sewing along and realize I ran out of bobbin thread 48' ago (after I've pulled out all the pins - because you're not *supposed* to sew over pins). Sew you pull out the quilt. Repin. Wind the bobbin, and pick up where you left off.
I have an indicator that tells me... but if you don't pay attention... It should be a really bright neon light... :-) :-)