After installing this addon, I noticed that the Lambda players would drive extremely slow when entering vehicles (this was tested with the Half-Life 2 jeep and Episode Two jalopy); upon exiting the vehicle, it would suddenly lurch forward and drive around erratically on its own for a bit before stopping. I disabled any addons I thought might be causing a problem, with no success.

After some more testing, I found that this occurs if you are playing sandbox on a locally hosted server, such as peer-to-peer - although I have not tested the other local host server options. The simple fix here is to play in singleplayer, but I like to keep a server running in case a friend wants to hop in.