This has got to be it (the above is speculation, not a spoiler).Conspiracy theory here, I actually think the memo was forged/planted by someone (even Hardman maybe?) to damage Harvey and the firm. Yeah, I know there are no real solid proofs on that up until this point, but it seems kinda strange that even after being caught and exposed Donna STILL claims she doesn't remember signing the memo or even getting the memo in the first place. Maybe it's a stretch, but it doesn't really sound Donna-like to forget things like this.
Edit: Spoiler tags just in case.
Hardman is obviously evil. But the show has been disciplined in not showing its hand yet - it's been half the season and he's still plausibly playing the changed man story. Hardman's been plotting something big behind the scenes all this time, and this was it.