See https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Thanks_not_working - ([[ https://en.wikipedia.org/wiki/Special:Permalink/826605511#Thanks_not_working | permalink ]])
When I tried to thank Davey2010 for his [[ https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&diff=826593042&oldid=826591709 | edit ]], it looked as though it went through. I hit refresh on the revision history, and it says I've thanked Begoon for their [[ https://en.wikipedia.org/w/index.php?title=Wikipedia:Village_pump_(technical)&diff=826602345&oldid=826601573 | 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 [[ https://en.wikipedia.org/w/index.php?title=Special:Log&action=view&user=MusikAnimal&type=thanks | thanks log ]] also shows Begoon was thanked, and not Davey2010.
I'm using Chrome on Mac OS El Capitan, Vector skin.
The thanks button on en.wp has been temporarily hidden to avoid disruption. [[ https://en.wikipedia.org/wiki/Special:Diff/826612112 | diff 1 ]] [[ https://en.wikipedia.org/w/index.php?title=MediaWiki:Minerva.css&diff=prev&oldid=826668152 | diff 2 ]]
@Trizek-WMF at 11:20 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 this are sleeping. Thank you for your patience.