I've seen these messages once or twice since buying my Tesla about six months ago. I'm not overly concerned with that frequency, but if it happened often, I'd take the car to Tesla for service. I'd also suggest taking notes, and maybe photographing the surroundings if possible (maybe save a TeslaCam recording), for a few instances. This might help the service center diagnose and fix the problem.
Just an FYI to consider, when I read your subject line ("Blindspot monitoring and Lane Departure avoidance errors"), I interpreted that to mean either that you were seeing false alarms (warnings about cars in lanes when none existed, or about lane departure events when you were tracking straight) or misses (no alerts about cars or events when they were present or happening). Without seeing what you saw, an "error" could be interpreted as just about anything. I'm not trying to pick on you or be pedantic, just use this as an illustration of the importance of clear communication when asking for help about a problem.