If there's a tab in the answer, and the part before the tab is changed, the words after the tab will appear changed.
Here's the issue in action on Bitcoin SE. The address looks changed, even though it hasn't.
The non-rendered diff shows this correctly.
I also created a sandbox answer to reproduce the problem. (Revision 15 is the only relevant one.) The first and last lines are correct, but the middle line incorrectly thinks that the third XXXX has been added and removed. This issue only shows up if the number of removed characters is not a multiple of four.
Screenshot of sandbox answer in case it's deleted: link