Reproducing it is very simple, in my test case (with video) I did the following steps:
- [optional to see result] Go into 3rd person mode with numpad Enter.
- Equipped an NLAW with mouse wheel and space (doesn't matter if the NLAW is loaded or not).
- Crouched by pressing X.
- Pulled out the binoculars with B.
- Once the animation is complete, you should be looking through the binoculars.
- Now move using the WASD keys and the player will glide along the floor with no animation.