Halp detecting invalid values

Lets say i have this series of values, representing speeds:
50,60,60,120,61,65
The 120 is probably a read error, so i need to detect it as wrong and discard it. I know that because is 60, then 120, then 61.

But what if i have
50,60,60,120
And i discard 120 as out of range, but the next value is 121, and later 125. Then the 120 is probably ok. So i discard 120, but then i realize it is ok and i accept it again.

What algorithm can i use for that?

>>4
I really don't understand your problem. What's wrong with queueing the last n values and doing filtering only on the middle ones, sending only the values which overflow the queue to the event-processing machine?
Sure, it adds latency, but you said yourself it's the best way.