REST API is serving content of current revision for old revisions. This affects at least visual editor and visual diffs.
For example: https://en.wikipedia.org/api/rest_v1/page/html/The_Fighting_Temeraire/1123182247 is identical to https://en.wikipedia.org/api/rest_v1/page/html/The_Fighting_Temeraire/1123185682.
1123182247 should contain the phrase "The painting currently hangs"; 1123185682 should contain the phrase "The painting hangs".
I originally noticed this because visual diffs were all showing "No changes", e.g.
https://en.wikipedia.org/w/index.php?title=The_Fighting_Temeraire&diff=1123185682&oldid=1123182247&diffmode=visual&visualdiff=1&diffonly=1