[hightreason]

Try playing without the HRP and see if it stops happening.
I imagine that it is happening in the time it takes to load the textures for the next area (ones you haven't seen in the level before that point)

When it happens, you just have to press the same keys or move the mouse the same way the player is going (Like if you are stuck going forward, press forward and it will stop when you let go of that key)

Like i said, it is just that the engine has not realised you let go of the key (if i remember rightly), so pressing it again and then letting go of it sorts it out.

Strangely (but completely unrelated), i work with MIDI files a lot, and it has a similar issue where it won't always notice that i let go of a note.