Cite backlinks are named with alphabetic characters (a, b, c, etc) when there are multiple backlinks for the same reference. When an article is opened in Visual Editor, these backlinks turn from alpha to numeric.

In the example given the two classes named mw-cite-backlink contain a b. When the page is opened in Visual Editor, the a and b turn into numbers (1.0 and 1.1 on the first line and 2.0 and 2.1 on the second).

The problem manifest itself on this page:
https://nl.wikipedia.org/wiki/John_Wood?oldid=43321289

Example screen shots:
http://gyazo.com/8413e498f4b11135e0e09a019f038c19 – this is the normal view
http://gyazo.com/18f9933ac72ba8dbd61e1ca2a2312832 – this is how it's shown in Visual Editor