if you use your main weapon or sidearm hit sprint shortly before holding mouse 3, you will be knifing and sprinting at knife wielding speed at the same time. I would post a video if i didn’t get suspended shortly after discovering it (already posted in ban appeals) 
This bug is actually quite fun (because its harder than actually just shooting). A melee class that could sprint and knife at the same time without movement penalties could be a really fun and challenging merc to play.