This is largely why I approach any potential problem at a table with a question first - it's not that I'm drilling or testing the players, I'm just getting them to stop and sanity check my observation of events. It usually helps them realize their error without my direct intervention, and/or illuminates some detail I missed. :)

My general rule of thumb is if a single word can prompt them to discover and fix a small inconsistency while I am watching a match (revealing the top card off Courser or placing a card into the graveyard instead of in exile) then I just let them play on. If me saying “exile” confuses them and I then have to explain something in greater detail I will issue a penalty.

For those saying if you can fix the problem with a word you won't issue a penalty, how would you handle being called to a table elsewhere straight afterwards where the same thing (say a player placed a flashbacked spell into the graveyard) happened?

Originally posted by Gareth Tanner:

For those saying if you can fix the problem with a word you won't issue a penalty, how would you handle being called to a table elsewhere straight afterwards where the same thing (say a player placed a flashbacked spell into the graveyard) happened?

It's a good point. The flip side is how many of them would have been called to you if you'd not been sitting there. People can get 3 warnings very quickly when it'd normally be fixed in the flow of the game if we hit them with a warning for every little thing we see. Not revealing for courser is a good example. 99% of the time, assuming that it's not got to a card being drawn unknown, the oppo will point it out without calling a judge.