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?

Just make a rule so that you can't discard a value from the last n ones, where n is greater than one and lower than the number of last values your interpolation method uses to produce a bound on new values.