When there's an uneven number of slices of bread so you need to get a new loaf to finish your sandwich...
O M F G I could not agree with you more on this. If you look at the back of the bread bag, sandwhich bread is 21 slices + 2 crusts and toast is 17 slices + 2 crusts. I can't vouch for 100% of the brands out there doing this obviously but it does seem that they all do it. Bloody annoying when you're on the last loaf when this happens and you live 10km from the nearest shop.
You could always cut the last slice in half !