I taught myself how to free-view stereo side-by-side images on my phone but after I did so I found that I would get strong stereo impressions involuntarily just looking at certain repeating patterns in normal circumstances like watching TV with side-by-side images or when looking at patterned wallpaper.

My visual system is pretty weird in general so I don’t know how common an experience this is with others. It’s not bothersome at all because I know what it is but it was a little startling the first few times.