I play skirmish against the AI.
Some of my replays have clouds of zombies. These appear to stream in and out of the enemy base around mid-game and will gather in my positions. They attack my passing units constantly, and sometimes those shoot back, but with no damage. The overall effect is to derail the replay, such that it no longer shows what really happened. It will end at the right time e.g. with the enemy Nexus destroyed, but with all my units somewhere else when that happens.
The attached pictures shows one of these zombie armies with units streaming in and out, and impotently attacking an Engineer for several minutes.
[Guessing... this might be because during the game I loaded an autosave to continue after a bad tactical mistake. At the end I saved the game for use as a replay.]
==> The replay feature needs attention - it constantly crashes on longer games, especially if I zoom in or out past mid-game. And the Barrager and Oblivion turret icons are a mess.
Zombies in replay
Zombies & Engineer