- Someone climbed out of the roller coaster. Don’t get out of a rollercoaster while the roller coaster is in motion. Stay in your seats. For example Space Mountain recently reopened after being shut down: a 20 year old man with cognitive disabilities climbed out of the moving rollercoaster and was on the tracks, forcing the ride to halt to a close. Thankfully, no injuries were reported. Somehow he was able to maneuver around the ride’s safety restraints. So lower those bars and hold on tight!
Rollercoasters have safety restraints for a reason.
- Elsa’s head may have fallen off, again (Frozen Ride). Do you want your toddler to see Elsa’s head off. Neither does Disney. Apparently the Frozen Ride opened in 2016, before it was completed, and the sensors are often tripped or the animatronics do not stay on. In these situations, all the lights will come on and you will be led out of the ride by a cast member.
The spinning teacup ride almost never shuts down.
- The boats ahead of you might be stuck. In Pirates of the Caribbean, for example, sometimes the boats can get so stacked up that the boat you are in CAN’T navigate the pirate waterways. Don’t worry, a pirate wearing galoshes may show up and use a long pole to navigate your boat out of the waterways. You should still be safe!
If you bring someone below the size requirement be sure to protect them. 
Slow moving rides on tracks mean that you have to get off quickly! 
buzz light year space ranger spin usually is OK, sometimes has electrical issues..
- Not too terrified to ride the Tower of Terror? Well, let’s hope it’s up and running for you! Before you discover life beyond the fifth dimension, let’s hope your elevator is lined up on the track’s properly. If it is even a 1/64th inch off (for your safety), the ride stops automatically. Take a look at these folks who filmed their ride being shut down.
- Last reason that a ride may be closed temporarily? Annual maintenance/upkeep. Check out this helpful list of upcoming closures for maintenance on wdwinfo.
Worst part of having a ride close mid-wait or mid-ride is the anticipation that comes to a slamming halt. Best part is that you will most likely be offered a fast pass by the cast members at exit. If not for the ride you were waiting for, it may be one that can be used for almost any ride in the park. If they do not offer a fast pass, be sure to ask for one.