Trigger based on Time Difference between two sensors / Catch a Miss on sensor
Use S1 to Reset an SR latch and trigger a 555 monostable circuit.  

The 555 one-shot will produce a negative pulse in response to the trigger.  AND this with S2, and use it to Set the SR latch.  The output of your latch is the error trigger.

This will malfunction if the S2 pulse ends after the monostable negative pulse, so you'd need to work the pulse-width into your design.

A better solution without that last weakness would take some extra logic -- perhaps using the 555 output as an !Enable input to another positive edge triggered device, triggered by S2, so the the rising edge of S2 cannot trigger.