You can't cast Vyper's ult while Holding crouch (Ctrl) + backwards (S). Strangely you can still use her Screwjab Dagger, Venom, and even snowballs. You don't even need to be sliding. Crouch walking backwards makes it impossible to throw your 4.
I found this out during a match where was sliding...
The video below shows it all. Essentially:
- turning an enemy hero into a rabbit and punching them is impossible, they're melee hurtbox is toggled off (suspected)
- this goes for both heavy and light melee
- you can still damage them with abilities and gun
- you must (as Sinclair) heavy melee...