Saw it here: https://www.mediawiki.org/wiki/Extension:DiscussionTools?veaction=edit

Placing the cursor selected the character one position to the right. This also happened when moving the cursor with the arrow keys. It was essentially impossible to create a collapsed cursor.

The bug was appearing very intermittently, but once it appeared in a session it appeared every time I moved the cursor.

Unless we can find a way to trigger this reliably, it may be hard to investigate, but filing this in case someone else finds the same issue.

It's not impossible this was caused by partially cached code, given it's Thursday, but the only big change to CE was https://gerrit.wikimedia.org/r/#/c/VisualEditor/VisualEditor/+/592398/ which was rolled out in wmf30 (two weeks ago?)