The flight was en-route from Washington DC to San Francisco when it was forced to make an unscheduled landing in Denver, where the passenger was freed upon landing.
United Airlines confirmed in a statement that flight 1554 was diverted to assist a passenger “who was in the lavatory when the lavatory door became inoperative.”
“The passenger was safely removed from the lavatory after landing, and customers have since continued on to their destination on a new aircraft,” the statement said.

P.S. Such situation can happen with a crew member as well. Here is the recording:

Lightning is an atmospheric discharge of electricity. A lightning strike can be very distressing to passengers and crew but damage to an aircraft in flight which is sufficient to compromise the safety of the aircraft is rare.

Effects
• Aircraft Damage. Structural damage to aircraft from lightning strikes is rare. Nevertheless, there have been many incidents of lightning strikes leaving puncture holes in the radomes and tail fins of aircraft and damage to control mechanisms and surfaces.
• Crew Incapacitation. Momentary blindness from the lightning flash, especially at night, is not uncommon.
• Interference with Avionics. A lightning strike can effect avionics systems, particularly compasses.
• Engine Shutdown. Transient airflow disturbance associated with lightning may cause engine shutdown.

A wildlife strike is a collision between an animal and an aircraft which is in flight or on a take-off or landing roll. Wildlife strikes may occur during any phase of flight but are most likely during the take-off, initial climb, approach and landing phases.

Effects
Wildlife strikes can be a significant threat to safety of aircraft. The impact of wildlife strike has been experienced to cause:
• Cracked or broken windshield and consequently, depressurization and possibly pilot injury.
• Engine failure due to ingestion, resulting in aborted take-off or emergency landing.
• Structural damage to the fuselage, control surfaces or landing gear.

Defences
The defences against animal strike include measures to:
• Perimeter fencing to make the aerodrome inaccessible to non-flying animals as well as measures to make it less attractive to birds.
• Observation of the manoeuvring area for birds and other animals and provision of information to flight crews. Information may be passed by the controller or using ATIS or BIRDTAM.

A Boeing 747 en route from New York JFK to Liege, Belgium, was forced to turn around on November 9 after a horse got loose in the cargo hold.
The cargo flight operated by charter airline Air Atlanta Icelandic had climbed to around 31,000 feet when the crew contacted Air Traffic Control in Boston to report that the horse had escaped from its stall.
The pilots requested to return to JFK Airport and, because the plane was too heavy, to dump 20 tons of fuel. The pilots also asked for a veterinarian to meet the plane upon landing.
The plane was able to take off again some three hours behind schedule. It landed in Liege at 6.49 a.m. local time on the morning of November 10.
This incident is one of the biggest-hitters when it comes to animals escaping on planes, but critter problems are surprisingly common in the air.