What I'm really talking about here is an overplay.
Specifically, I'm talking about the situation where you are doing ok locally or globally (not losing terribly).
You are considering either a response or a sente tenuki move, and there is one or more sensible, safe moves that would yield a reasonable result.
But, you want more. Maybe just a point or two, or maybe you are trying to kill or split an enemy group where you really don't have the strength to do so.
So you play just a little bit further than you probably should.
And you get punished for your greed, by being forced to fall back, lose your overplayed stone, or have it royally backfire, costing you dearly.
Now, there are situations where being greedy and playing further than the safe move:
1) You are severely losing (losing by a lot in the beginning or a little in the end... more than you may hope to make up by playing safely).
2) You are the stronger player and have given a handicap.
Really, these are the same situation: if you've given a handicap, you are already losing on the board. More importantly here, you can expect to launch into wild dangerous plays and probably succeed due to the skill difference. More on this in a future post.
My big problem lately is a greed issue: I keep making gigantic moyos that I can't possibly defend. So, instead of having a secure, decent sized moyo, I get greedy and try to take a huge amount, and inevitably fall to pieces.