Add a new section with a wikilink in it. Say ==[[User:Jayjg|Jayjg]]==. Add a __TOC__ tag, then click on the Jayjg section in the TOC. The URL that is automatically generated in the edit history becomes http://en.wikipedia.org/wiki/Randompage#User:Jayjg.7CJayjg instead of http://en.wikipedia.org/wiki/Randompage#Jayjg. Editors cannot jump to the page when they click on this link. This same problem happens when you add a new section to the talk page with the wikilink in it.
I've also noticed a problem with having duplicated heading titles. The second heading title in the TOC, when clicked, jumps to the first section name.