I just had an assasination type mission, in which I encoutered infinitetly spawning enemies without the presence of a Warp Gate. This made actually figthing the heretics completetly pointless and eventually I decided to run for the last remaining target (the first two were close by), but I ran into a mob too thick run trough and died in a hail of gunfire.
This is the first, and so far only, time I have encountered this bug. Unfortunatetly, I have no idea what triggered this or how to reproduce it. The mission was a Chaos assasination on the planet in the Chernobog system on the outdoor map.