Random Elements Become Unselectable

It seems to me you are working in a 3d file. Your Active Depth may be different than your View Depth so essentially when you are moving an element the data point is actually a Z coordinate higher than what you can see. When you zoom extents you can then see the element. You may be able to select it using Power Selector and crossing mode rather than being able to actually just click on it to select it. That's because your cursor is at a different elevation. If you do not need to be in 3d export the file to 2d and that should fix your problem. Otherwise you need to become familiar with changing Z depths and view depths. There may also be front and back clipping planes affecting this. Can you give more detail about the file?

Not a bad thought, but we rarely use 3D files and the files in question are 2D. I can't see any correlation between the files. Different projects, different seed files, etc.

Are you able to post a sample of the file see others can evaluate it to look for problems?

The next time it happens in a file, I can post it for review. Since it happens periodically with different files, I can't say for sure which one it has happened in. I will wait till I know for sure.

The next time it happens in a file, I can post it for review. Since it happens periodically with different files, I can't say for sure which one it has happened in. I will wait till I know for sure.