Why did my OB fail during the observation?
Although ESO carefully reviews OBs before they are scheduled at the telescope, it is often impossible to detect an OB problem until it is actually executed. If an OB fails to execute due to an error introduced by the user (as for example if the user entered wrong target coordinates), ESO reserves the right to charge the respective observing programme for the time lost during night operations. Time charged will include any telescope time used to investigate why OB execution failed.
For service mode observations time lost due to failures caused by ESO (e.g. instrument malfunction) will not be charged to the user. Whenever possible, these OBs will be re-executed as time, conditions, and overall priority permit.
In case of a time critical OB, the OB will have status F(ailed) if the time window expires before observation. You can find more details about the policy for time critical observations at this link.