When I tried to thank Davey2010 for his edit, it looked as though it went through. I hit refresh on the revision history, and it says I've thanked Begoon for their edit. I tried a second time, and it thanked Begoon's other edit.
It seems it is thanking the most recent non-thanked revision on the page.
My thanks log also shows Begoon was thanked, and not Davey2010.
I'm using Chrome on Mac OS El Capitan, Vector skin.
@Trizek-WMF at 14:50 UTC:
- All wikis are impacted, whatever the browser/OS; whatever the skin (mobile included)
- All thanks are shifted: you thank the previous edit, not the current one.
- Early investigation shows that DOM is correct, but the API is somehow malfunctioning.
- Developers who can possibly fix have submitted a patch that needs to be reviewed. Thank you for your patience.