Yeah you will fark your knees pretty bad if you let that plate shift laterally beneath you. Its happened to me several times where I start to complain (in my head) of some knee soreness 100km into a ride and when I get home the cleat plate had shifted. I love my speedplays but you have to be fairly vigilant. They stress not to over tighten the plate screws but I have long since ignored that.

Tighten the barstids good and proper. Especially if you are throwing down sprints in a session.