If a zombie begins to break a door but the door is removed before it's done, it will continue to try to break the block in that space. The door breaking sounds will continue to play even if there isn't anything there anymore, and the door particles will appear when the door would have broken.

If the door is replaced with another block, that block will be broken instead.

This video by SimplySarc goes into a bit more detail: https://www.youtube.com/watch?v=-LLMC26_T78