Precondition Failed when saving snippet - means what?

I get this error and Modx refuses to save changes to snippets. All I’m trying to do is duplicate (with a new name) an existing snippet (UserComments).

Modx0.9.2

I had a similar problem a while ago... it was linked to a corrupted database.
Did you recently move your site or import a backup or something ?

Anyway, for some reason in my case the DB was imported but some fields were not set to auto_increment, I edited those comparing with a clean MODx install which fields had to be auto_increment and that solved it.

In your case, I’d check that ID of modx_site_snippets has the "auto_increment" property... I hope it works for you...