A list of (probably) all posts that show this problem is returned by the following query
Which returns 11554 postings, this shows all the pre-2014 postings on the site whose initial unedited markdown has a single backslash followed by four spaces, or spaces and a tab. This is a (very) small over-estimate of the posts corrupted. A few tens of posts had ASCII-art diagrams involving
\␣␣␣␣ and a few less than that had
\␣␣␣␣ in actual tex code.
So fixing this problem meant applying a regular expression replace of
This could have been applied in about a couple of minutes by anyone with write access to the back end.
Initially a different search was used in SEDE (and similar searches using
code: in the live site web search)
which currently returns no results.
This shows all posts prior to 2014 with HTML with a single backslash followed by a tab or four spaces (tabs are are normalised to spaces in the SQL data dump HTML)
However in March of this year, after several posts had already been fixed before this data query was produced, this was showing 7322 posts, as noted in a comment under m0sa's answer.
As will be explained later, this missed over 4000 corrupted posts, but this was not realised at the time.
A purely automatic method of fixing these posts was complicated by the fact that there were some (less than a hundred) posts with legitimate use for backslash-four spaces. Typically "ASCII art" renditions of trees. For postings prior to 2014 as returned by the above query these have all been changed to use a form not returned by the query (using non-breaking space for ASCII Art or just formatting the tex code not to have four spaces after a single
\) This was done to prevent the same postings re-appearing and requiring human intervention each time anyone tried to search remaining bug instances.
As Stackexchange failed to provide any tools to address this issue, the bulk of the questions were addressed by running variants of the above query, which provided links directly to the edit page of the affected post. Then copying the text to a buffer in an emacs text editor which had the required replacement function already defined. This function fixed the text and put the replacement text back in the operating system clipboard in a few keystrokes, which could then be pasted back into the browser edit window and saved.
The emacs lisp used query-replace so that the edits could be checked for false positive cases as described above, these had to be handled by hand, but for the bulk of cases a posting could be fixed in around 10 seconds. For ~7500 postings that is still 21 hours, and actual elapsed time was certainly greater than this as people slow down, or the site flagged issues with images in edited posts that required manual intervention, or any other reason.
The data-query and emacs based workflow described above was used by Moriambar and myself, Barbara Beeton who was the other person to do bulk edits used a variant using site search directly on the live site (apart from anything else having the alternative search helped fix some issues in our data queries). Many other people helped out by fixing smaller numbers of queries, in particular fixing their own posts that had been affected.
After the first version of this answer was published it was realised that the search on the live site or the data-explorer post table is checking the pre-rendered HTML, The SQL was modified to instead check the last edited entry in the posthistory table which records the underlying markdown.
This link does check the markdown (by looking in the post history)
Initially produced 4573 missed posts, and after a couple of days when the SEDE dump was refreshed on the 18 July this was down to 790, and I believe these are now fixed, so it should be 0 next week.
A list of everyone who edited a pre-2014 posting in 2017 is:
It's worth noting that by far the largest number of edits is by "Community" with 3 times as many edits as the next person. These would (mostly) be edits to change URL from http to https. Note this was done as a silent back-end edit to the collection, without flagging each question as an edited question and raising to the top of the home page, and without requiring dozens of hours of manual volunteer work to make the change. It is a shame that Community could not have been coerced into fixing this issue. (Bulk editing the full list followed by a manual check for the few cases where the edit was not required would have been much quicker).
Note that the "2nd batch" of 4300 posts looked OK on the site
but the underlying markdown was corrupt, so any edit to the question would put corrupted text with missing
\\ into the edit box.
This issue is the reason for the related question on meta
‘double backslash + newline’ collapses to ‘single backslash’ when I hit ‘edit’
It is also the reason why the issue surfaced particularly strongly this year, three years after the initial corruption took place. Earlier this year Stackexchange edited all postings with images replacing http URL with https.
On this site that would have re-generated thousands of old posts that were visibly OK but had corrupted markdown, so exposing thousands of corrupted postings on to the public site.