This appears to be an issue with ShadowBox. I removed the ShadowBox plugin, switched to ThickBox, and although I preferred ShadowBox, this resolved the problem for me.
I am also having this issue. Each time I try to place a thumbnail image from my media library (any image) into any new page (not a gallery page, just a normal WordPress “Page”), I get this appended to the text of that page: ;player=img;” And, an expanded gallery just automatically shows up on the page.
