I saved a copy of the drawing to troubleshoot and started deleting things until the issue went away. It didn't so much go away as it did not pop up. No matter what I did, the errors stayed in the window. But if I closed the drawing and re-opened it, they would not re-appear. For some reason, what eventually made it stop was deleting one of the parts from the drop-down list on the View Palette. The first and second parts are on the first sheet, the assembly and the third part are on the second sheet. If I switched to the first sheet, brought up the third part in the View Palette and deleted it, then closed the drawing and re-opened it, the errors didn't pop up. Note that all the views of the actual part are still there on sheet 2. Any one care to explain how this makes sense?
In drawings, each view and each sheet is also a sketch. If there are sketch entitities (most often centerlines) drawn on a view, they are part of that view's "sketch".
Use "lock view focus" to gain access to the view's sketch and view the constraints of each view.
For some reason, sketch constraints are not well-displayed in drawings.
Thanks for the advice. I'll keep that in mind for trouble-shooting future problems. I was thinking that there was something wrong with a sketch on a view, but I'd have thought deleting the entire sheet would have made that problem go away. It seems odd that just deleting one of the models from the view palette was what finally stopped the error from popping up.