BEPUphysics

I got most of the details It happened on a collision between the player controller and an arrow, on the collision I create a weld joint.

Here are the details
http://i.imgur.com/ezm7l.jpg

That usually happens when adds and removes are performed from a multithreaded context. If the WeldJoint is being added from an immediate event, move it to a deferred event or add it completely outside the execution of the engine.

If it is something else (perhaps related to multithreaded characters), moving to the latest development version would likely help.

If the above isn't it and it still happens, I'll probably need a debuggable reproduction case in the demos or something to figure it out

I like this forum, amazing and very informative,