This bug has been in the game since the Inertia update, in which movement around obstacles or infected, especially when ping is moderate to high, and especially when going down stairs, causes jittery, desynced movement and in some cases the appearance of clipping through buildings. I think it's likely due to client and server calculating movement differently. This has got me killed a few times and has remained unfixed for a long time considering its severity and ubiquity. Likely the largest bug in the game currently.