Impossible timing in a measure?

I was composing something for piano and somehow ended up (in 4/4) with a whole note followed by a sixteenth rest in a single bar. I'm not sure if this is possible and I just don't know how things work for piano (more of a flute guy myself). I've tried deleting the rest, tried changing the whole note length and back again, tried copying a whole note over it, and also have checked and verified that the measure duration in the measure properties is 4/4 nominal and actual. If it is somehow possible in piano music, I still don't want it either way and help would be greatly appreciated :)