There seems to be an issue when saving content of more than 65,535 characters.
65,535 is the length of a TEXT field, even though the introtext and fulltext fields are set to MEDIUMTEXT in the database.

The error is:

Error
Save failed with the following error: Data too long for column 'description' at row 1

Once that error has happened, stuff gets broken in the database and it is no longer possible to correct the issue by deleting content. When you try to save again with a shorter text, it will throw this error:

Error
Save failed with the following error: Incorrect integer value: '' for column 'hits' at row 1

It took some digging, but the issue is down to the Finder plugin.
This gets triggered on onAfterSaveContent. And tries to save the full content to the description field in the finder_links table. That field has the type TEXT. So it chokes.

So not only is there an issue with that field length, there is also an issue with error handling.
It shouldn't be allowed that if a plugin has an issue with saving meta data, it causes the initial content (article) to break and go haywire.