If you enter an invalid label name (invalid.label) when editing a page, you are correctly returned to the page edit but the label section is hidden. You can hit submit as many times as you like, you will continue to return to the page edit screen with no error message shown.

If you open the label section you are correctly presented with the error.

This issue was introduced some how during 2.6 development as its not appearing on 2.5.6.