"... 1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. c3 Nf6 preparing to castle, ..." -robbie_1969
"4... Nf6!" - mickynj
"4...Nf6! ... The knight attacks the pawn on e4, which can no longer be protected by Nc3." - First Steps 1 e4 e5 by GM John Emms
https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf
I think it was move 6 or 7. You moved Be6. Made him able to capture your Knight and forking your rook. That was biggest blunder that i saw.