Board Thread:Plants vs. Zombies 2: It's About Time/@comment-25702176-20150302050314/@comment-24765950-20150302160335

Milesprower2 wrote: Frozen Imps near the lawn wouldn't cause this to happen, as frozen zombies at the end of a level die immediately.

This is simply a Troglobite stuck off-screen. Sometimes frozen zombies fail to die after the level ends. It's a rare bug though, and I've only ever seen it once.