Coaster loses a washer?

Doesn't a nut or bolt have to come off before a washer does?

Not if said washer was never part of a bolted joint in the first place, or if it broke.

Did you see this washer near track on the ground? A lot of the time when they use weight in the form of a sandbag to get a train warmed up, the sand is actually old washers or nuts. Occasionally, a bag will break open just enough to let some washers fly out.

I assume this post is about a video posted on Facebook about someone seeing a washer fall off a train on Magnum recently. I believe the person picked it up and brought it to staff to notify them but not before recording a video and making a big deal about it. The ride was closed for an hour or so while they inspected it.