Steps to reproduce:
- Go to https://commons.wikimedia.org/w/index.php?title=Special:Upload&uploadformstyle=basic
- Scroll to the end. See (one) "Categories (?) (+)".
- Scroll back up a bit and [Browse] for a Source filename.
- Scroll to the end. See two "Categories (+)" (but only one blue (?) button).
- Scroll back up a bit and set the destination file name.
- Scroll to the end. See three "Categories (+)".
- Scroll back up and change the destination file name to something else.
- Scroll to the end. See that yet another "Categories (+)" has been added now.
- Repeat the last two steps as many times as you feel like.
Use one of the extra Categories links to add a category. Upload the file. Discover that your cat wasn't added.