Seeing the errors of other Players

[AmericanAirlines]

Hi Everyone,
I've read that, "If you can't see that other players are making errors, you probably are the fish".

So what errors do you folks commonly spot? What are you looking for exactly?

I've been thinking about my play, against my read of the opponent. Have yet to seriously consider what the mistakes of an opponent are... because I don't neccesarily know what all the errors are, or what correct play in every situation is.

Sincerely,
AA