Further examples of pages that have disappeared due to database corruption:

Backups can be used to restore lost data, not just revert. Revs should not be disappearing from the database. Nor should the problem be getting worse with time.

In T298#4466, @--miki-- wrote:

This seems to only affect certain pages, specifically those edited between July and last week, as I was able to edit my user page and a template today.

The founder of rudubbing.shoutwiki.com reports the same problem.

It does appear to be fixed. Thx

BTW, there appears to be no such thing as a #security tag in this phabricator ....

In T144#2129, @lewiscawte wrote:

In T144#2122, @Saftzie wrote:

It seems I'm not alone.

That has been known for a while.

Hmm, it's worse. Log in to Cook's test wiki.

It seems I'm not alone.

FWIW, I just verified T146. Viewing a global Help: page logged me out, too, although the Help: page did display.

Log in to Cook's test wiki. There's nothing special about the wiki. I t can be any wiki.

I delete all my cookies every time I close my browser, so every session is a fresh session. It's not a cookie issue.
Screenshots:

The error no longer occurs.

In T124#1700, @CJC wrote:

It's in the i18n files - "createwiki-newmainpage" is the message. Will need to be changed in all language files.

In T124#1708, @SamanthaNguyen wrote:

...
em units are used because they're relative ...