I was in the Render panel and I had a layered by brackets panorama. I typed %l into the filename template but hit cancel. I opened the Render panel again and now all the image had the bracket value (e.g. -2.0) in their file names, as if I clicked on apply instead of cancel, but they also had the red exclamation marks, as if %l was missing (and it was missing, so the filenames shouldnt have had the bracket value appended).