Here is a compilation of 15 positions from my games (Online and OTB) where my opponents made critical errors in the opening. Some solutions are obvious, and some are not so obvious. These are made in no particular order:
#1: In this Sicilian Taimanov, white just went "danger levels", and the black queen looks trapped. However, black has a strong refutation here:
#2: In the Petrosian System of the Grunfeld, white just took the "free" pawn on d5. Is black going to just lose a pawn, or is this move a mistake?
#3: In this Taimanov Sicilian, white has just played f4 to gain some space. Is this a good move, or does black have a tactic?
#4: In this Evans Gambit, black has just castled his king, thinking he's safe. Is he really though?
#5: Black has just played d6, blocking the bishop's diagonal and thinking he's safe. Will white just be down two pawns, or does he have a tactic here?
#6: Black has just played Nde4, protecting against mate and it looks like white has no more attack. However, white has a strong move to completely shatter black's resistance...
#7: In this Scandinavian, black played Qc6, keeping the queen out and expects white to develop. However, white can win the game immediately. How?
#8: In this Philidor defense position, black has played Bg4-Bxf3. Will this be another normal game, or can white prove a significant advantage immediately?
#9: In this KID, white has just castled, thinking nothing of black's obscure-looking last move: Qe8. However, black has a tactic to immediately get an advantage. How?
#10: White has just played Ba4 in response to a6 in the Rossolimo Sicilian. Black can now win a piece; how?
#11: In this sharp line of the King's Gambit, black played Qg4 in response to Nf3 by white. It looks like the queen will hang around and cause problems, but is the queen really safe on this square?
#12: In the Fried Liver Main Line, black has just played the innocent-looking Bd6, preparing to castle on the next move. However, this allows a tactic by white:
#13: In this Sicilian Taimanov (again), white just played g3, intending to punish Qc7 by Bf4. However, this leads to disaster for the white position:
#14: In this Traxler Counterattack position, black has just played Qf6 in response to g6, saving the queen and threatening Qf2# once more. However, this is the wrong approach, and white is now winning:
#15: This one should be quite obvious, yet I still see this blundered at the 1800+ level, and I don't understand why; black to play and win:
I hope you have found these educational. Hopefully this can help you recognize some of the blunders your opponents are making and to take advantage of them. GL in your games!