There are 35 anomalies which can range from the obvious to quite subtle, so you have to examine the corridor with a fine-tooth comb. If you spot an anomaly, you have to pull down a lever and start again, simple.
Except, every time you get it correct a counter raises - get to 13? You've escaped, and are rewarded with freedom, get one wrong and the counter resets back to 0… and you start back over, that’s right, 13 anomalies in a row, or that comb is coming back out!