An unintended mistake of some kind.
(a) Befouling oneself with something vile (see shart
(b) Getting caught doing something stupid at work that results in a drug screening.
At the urinalysis drug testing facility:
Nurse: Did we have a little accident?
Patient: Accident? *looks down at self with bewilderment*
Nurse: Oh, you must be here for pre-employment screening then.