This "?contents=1" also show the web parts which were added to page in past. Go through the below given images to have an idea what i am trying to explain.
- Here you can see a page with 4 web parts added to it.
- Quick Clicks - This is my custom webpart. This webpart contains an error which blocks the whole page to load.
- Image Viewer - OOTB web part.
- XML Viewer - OOTB web part.
- Content Query - OOTB web part.
- When I clicked "Save & Close" from the top ribbon. The page bombs out giving the error mentioned below.
- Now as i mentioned above i used "?contents=1" and it showed me the list of all the web parts for this page.
- In the above image "Custom Web Part" was added to the page but was deleted, still it shows up in the history.