Last night, an alleged FSB whistle-blower letter was published that damned Russia's military performance in Ukraine and predicted a disaster for the RU in the next weeks and months. I wasn't sure if it was authentic - as Ukraine had previously leaked fake FSB letters as psy-ops.

This letter appeared different though: it came via a reputable source (founder of http://gulagu.net ), and it was way longer than a forger would choose to make it (the longer the text, the more risk of making an error).

I showed the letter to two actual (current or former) FSB contacts, and they had no doubt it was written by a colleague. They didn't agree with all of his conclusions, but that's a different story.
Here's the text, worth reading: https://www.facebook.com/vladimir.osechkin/posts/4811633942268327