In regards to "softening", eg:
>"Hey, you got your math on example X wrong... I think it actually works to 11.7. Anyways, I only recognize that because I made that mistake dozens of times myself, it's a common one to make, just wanted to point it out."
Stuff like that always reads as condescending to me. Not just a waste of time, but an expense of time which actually hurts ones cause. I suspect my feelings are neither unique nor universal, and that delivering respectful and constructive criticism involves knowing your audience. So it's always going to be difficult if you try to do so to a stranger.