I had an issue with a UV filter and a circular polarizing filter at a car show this weekend. I was using the 24-105mm L on a 5D Mk III, with a B+W F-Pro UV filter on it. A car I wanted to photograph had some distracting reflections on it, so I put a Hoya PL-CIR filter onto the UV filter. The polarizing filter was stuck when I went to take it off. I tried several times to get it off, but it wouldn't budge. I ended up shooting the rest of the car show with the polarizing filter on, which was inconvenient because of the loss of light and there was vignetting when zoomed wider than about 35mm. With 20/20 hindsight, I should have taken both filters off and crossed my fingers that the lens hood would provide enough protection from the crowds at the car show.
Attached is what the view was like at 24mm with both the B+W F-Pro and the Hoya PL-CIR on. I had to remember not to go wider than 35mm. I was able to get the filters apart later with a filter wrench.
I'm not sure what caused the Hoya to get stuck so badly. Was it the filter's aluminum frame? Was it the high humidity? This was my first time stacking filters since I started using a full-frame body, and now I know how easy it is to cause vignetting. I just measured the Hoya, and it appears to be 1cm/10mm thick. Considering how rarely I use CPL filters, I thought I would save money and go with Hoya even though all my other filters are B+W. Looks like that may have been a poor decision. The thin B+W polarizing filters are expensive, but I may end up getting a 77mm since it is the size I use most.