[FEN ""] [StartPly "12"] [Title "What is wrong with 6..e5?"] 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bc4 e5
Please do not reply that it weakens the d5 square and allows the white bishop to attack f7. I am an expert player with lots of experience with the Open Sicilian from both sides in many variations. In view of the systems where Black plays e7-e5 even allowing the doubling of pawns after bishop takes knight on f6 it is not at all evident that this move is bad.
There is a related question about 6..e5 after 6.Bg5 without satisfactory answer for two years: Why does 6.Bg5 prevent 6...e5 in the Sicilian Najdorf?
The 2013 book on the Najdorf Sicilian by Zaven Andiasyan is a Black repertoire book which replies e7-e5 to anything except 6.Bg5 and Bc4 without a single line of explanation why not.