Error Code

This Error keeps showing up and it says this:We’ve run into an error. This may be due to accessing this content from multiple locations. Please close other locations, then return here and refresh. If this problem persists, tell us. [Error code: STATE_MISMATCH:NjM0YzAwZWYtYTM3Mi00Ym]. The problem is that I only have one Khan Academy tab open and I keeps showing up on my screen while I'm trying to do the math exercises.