This is the 13605th most frequent Russian word.
недопустимую
"Unacceptable."
In this case, "недопустимую" is used to describe the temperature being out of an acceptable range.
Профессор отметил недопустимую температуру в лаборатории.
The professor noted the unacceptable temperature in the laboratory.
Here, "недопустимую" is used to describe behavior that is not allowed or tolerated.
Такое поведение — совершенно недопустимую.
Such behavior is entirely unacceptable.
"Недопустимую" modifies "ошибку" to indicate a critical mistake that should not have occurred.