After the new 'last stand' update, I began noticing errors with some of the textures on some of the common infected. In addition to that, my flashlight wasn't working properly.

After disabling this addon that I must have subscribed to a few years ago (and subsequently forgot about), I stopped experiencing these issues.

I hadn't played L4D2 in a few months. It could have been an error that was introduced between then and when the last stand update came out, so I cannot say with 100% that this is attributed to that update.

However if any of you are experiencing an issue that sounds like this, try disabling this addon and see if it helps, it did for me :)