I always use either a UV filter or no filter and the lens hood, depending. Today I had the filter and no hood on my 28-135 IS lens.
I dropped the camera on the ground, lens first. The filter shattered and is sort of fused to the lens cap. The lens seems OK, although it has some broken glass on it from the filter. I'll send it to Canon for cleaning and it should be fine.
Here is the filter after the fall (taken with a different lens):
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