Marking a change in time measure
Let's say I start a piece with a 4/4 time signature. After a few bars I decide to switch to 3/4 and then in the very next bar to go back to 4/4 again. And after that I notice that I made a mistake, because that measure should have been 3/4, and only the next one should have been 4/4. If I go back to the one I've already marked as 4/4 and change its time signature to 3/4, that bar will be marked as 3/4 again, even though it continues in the same time signature from the previous bar. See the picture.