Not the first time it happens on the site, so it's time to raise the problem in Meta.
It is similar, but distinct from ‘double backslash + newline’ collapses to ‘single backslash’ when I hit ‘edit’ (Nov 26 '10); what happens is that code with lines ending with
\\ gets randomly transformed, without any intervention, into a single backslash and the following line becomes a continuation of the line previously ending with
The code for sure had double backslashes when it was posted.
See the edit history at https://tex.stackexchange.com/posts/146730/revisions (Nov 24 '13) for the last time I was made aware by it in some code of mine, look at the comment How to typeset row operations on augmented matrix
Another case I remember is How to correctly format (and align) a LaTeX proof? (Sep 2 '11) where my answer was fixed by Andrew Swann and, when I went to see what edit had been done, I discovered the same problem in azetina's answer.
Here is another example: How to align a set of multiline equations (Feb 13 '12) (a second degraded answer, by Werner, fixed; Feb 13 '12; except that the intended fix isn't present, but fixed now)
Vertically align cell table in figure (Dec 2 '12)
Set math fonts to predefined color (Sep 4 '13)
Using \gather and \align together (Apr 3 '12) (all answers, but not the question, are affected)
Label rows of a matrix by characters (Jun 13 '12)
Hyperref TextFields dont't work when in tabular environment with xetex? (Sep 26 '13) (the question)
Commutative Diagrams (May 23 '13)
How to create alternating rows in a table? (Nov 14 '10) (an answer)
eqnarrayto match that of
align(Jan 31 '13)
Table overlay in
beameron multiple rows (Nov 21 '12)
Rotated column titles in
tabular(Oct 25 '11)
Diagram of an artificial neural network (Sep 9 '13)
cases? (Nov 1 '11)
TikZ matrix as a replacement for tabular (May 18 '11) (both question and answer)
In view of the provisional answer given below, please only add examples that appear to be affected after February 22, 2015, below this comment.
Latest date (so far) of corrected example: December 20, 2013: https://tex.stackexchange.com/a/150816/579. (Please update this date if a later example is found. The aim is to pinpoint the likely period in which the offending change was applied in order to determine the cause.)
(empty, for now, hope it remains so)
Update: April 27, 2017
Nick Craver seems to have found the origin of the problem. While he blames himself, I think we shouldn't put him at the wall and fire.
The proposal he made seems quite appealing, particularly if the items edited while in the specific review queue aren't bumped to the top page. With all candidates collected in a queue, we should be able to get away with the issue soon.
An additional request would be to have two buttons: “Fix” and "Pull off the queue”, the latter for posts that are just false positive.
A good search criterion for a post being included in the list seems to be
it contains a backslash followed by four spaces or a TAB, and has been posted before 2013-12-31