On the map greystone plaza, sometimes enemies get under the map and keep walking under it towards the crystal.
Seems to be tiny skeletons.
I don't really know anymore than that.
However, this user on the steam forums describes his experience with the issue:
Today some enemies made their way under the map. There were 4 mobs left and i was walking around and collecting loot, after 30 sec i thought they might be stuck at the spawn but then i realized it was at the objective. Slightly panicked i ran over and couldnt find any mobs. I attacked with melee attacks then i realized that the enemies are underground (they started attacking the objective after i attacked them) from the sounds they made i assume that they were revived mini skeletons.
Besides another user confirming that they've only seen it happen on that particular map, there wasn't anymore info on it.
source: http://steamcommunity.com/app/236110/discussions/0/350541595117439970/?ctp=2#c350542683208197098
I farmed this map quite alot and this happens commonly. Its always skeletons and my guess is that Malthius Boss spawns them but they end up underneath the core. Its always the same spot and same units.
This has been a reoccurring bug for a while. Do you know what lane the enemies are coming from? Can you possibly get a video or a screen shot of this happening? Thanks.