Barnet boss Lawrie Sanchez believes Southend's controversial second goal was the turning point in their 3-0 defeat on Tuesday.
Ryan Hall and Bilel Mohsni came up with first-half goals before the Bees' Darren Dennehy was sent off in the second half. Neil Harris finished things off five minutes from time in the League Two clash at Underhill.
The home fans, who saw Izale McLeod fire an early penalty over the crossbar, raged at referee Andy Woolmer after he initially disallowed Mohsni's back-post effort for being offside, before reversing his decision after consulting his assistant.
Former Northern Ireland boss Sanchez said: "I've spoken to the referee and he said the linesman gave offside. The referee blew and gave offside. They've then conspired between the two of them to give a goal.
"That decision turned the game. I don't see how the linesman has given offside, and then the referee has blown for offside and then they've changed their minds. I don't know how that's worked."