"Card was modified" error message

Originally reported on https://forums.ankiweb.net/t/error-message-error-card-was-modified/51296/11

I suspect it's related to learning cards becoming due, but nobody has figured out reproduction steps yet.

Start with the linked forum report and the issue's description of the "Card was modified" error. Investigate the learning-card path around cards becoming due and first establish reproducible steps; done means the cause is fixed and the error no longer occurs in that scenario.

Written by the indexing model from the issue text.