What's the most frequentlyl mis-played opening by amateurs?

Why is 3...Bg4 bad in the Philidor?

The Ulvestad, while not played much, is misplayed by amateurs nearly 100% of the time.

Bg4 in this philidor position gives white an enormous advantage, as shown by Morphy in his famous "Opera house game".

I here you can blunder an entire bishop in the Cambridge Springs but no one never shows me how that can happen do you know ?

thanks

Because 4. dxe5 gives White a definite edge: taking back with 4. ... dxe5 is obviously a blunder as

wins a pawn as well as taking away Black's right to castle, and on top of that immediately followed up by the threat of Nxg4 as well as Nxf7+. The correct continuation:

gives White at least a great initiative and the bishop pair in an open position. And there's a nice trap: 6. Bc4 threatens mate in one and the "natural" 6. ... Nf6 allows 7. Qb3, winning at least a pawn (but in my blitz games often enough either a rook or even mate).

I saw you talked about Philidor but I didn't find about quick mate in it.
I checkmated some people with this:

Quasi-Pirc...

That's Legal's Mate :) I think I never actually got to play this (but in the Philidor I always play the mainline 3. d4). I think I won one blitz game with the 3. ... Bg4 variation like this:

Oh, Legal mate, of course! :) I forgot!

Here is one   1. e4  d5  2. e5?  and Black is better already.

thanks thats the trap I was looking for.

i have seen some players as black play 1.e4 e5 2.Nf3 Qe7 or ...f6 ,i know f6 is bad,but don't know what white should play exactly,i usually play Nxe5 followed by Qh5,i lose most games in middle or end games

I think I saw it once...