Saying something is
always the right thing to do. It's simply a matter of how to say it, who to say it to, and when to say it.
The 'who' part is really easy. You tell a judge. Don't interfer with the players while they're playing the match. Tell the judge about the problem you see, and let him do the interferring. Also, just because the judge is watching, don't assume that he will catch everything. Judges can and do miss or forget things, just like everyone else.
The how and when to say it can sometimes be a bit more complicated. Tell the judge away from the match, and generally wait until there is actually something wrong. In the given situation, for example, there isn't actually anything wrong until the phase ends and the players forget to notice the mana burn.
On the various reasons mentioned for not saying something, if you're taking actions based on whether they will hurt or help your teamate (or not taking actions), you may be helping someone cheat, or even cheating yourself. (Sure you may well not get caught and therefore not penalized, but cheating is still cheating)
As for choosing not to say something to avoid affecting a game you're not playing, there are certain other CCGs (specifically those made by Decipher

) in which no one, not even a judge, may interfer with a game unless requested to do so by one of the players. Magic is not such a game. If you see something wrong which everyone else misses, you
are affecting that game. Whether you say something (the right choice) or choose to keep silent (not the right choice), you are having an effect on the game.
While this is correct ethically, in my opinion, this is not a matter of ethics, it's a matter of rules. If you don't feel comfortable with the idea that you may cost a friend or teamate a match due to pointing out some error the players missed, you'd do best to not spectate at all (as Ephraim chooses), but there are two sides to that coin. By not specatating you may also not be there to point out something wrong which would allow your friend to win the match.
I said nothing for one reason, then kept silent because others kept silent and the judge was there too.
Is that wrong?
mentioning the drain mana seems like a no-no.
mentioning it after the fact A) Hurts a teammate B) makes me Mr. Smartypants and C) has me affect them game I am not playing in.
Now, pointing out that a card goes the the RFG zone when Leyline out is one thing, but mentioning Drain Mana which could be advantageous (pre combat) vs Dis-Advantageous (post Combat) all while being a teammate makes me wonder....