Replaced derailleur cable and housing. Shifting on my previous ride was noticeably mushy, and I know from past experience that this is a good sign that the derailleur cable is on its way out at the shifter end. Opened up my brifter and sure enough the strands of the cable had started fraying. Probably wouldn't have broken right away, but likely within a couple of hundred kilometres at most. Shifting is much smoother now, of course.
Already had >4000km on the cable, so it's not like it failed early.