Derby looked on their way to all three points after a brace from Tom Ince and a strike from Chris Martin at the start of the season half had them in cruise control.
However, three goals inside eight minutes saw Neil Warnock’s side come back to claim a draw in South Yorkshire.
Danny Ward pulled a goal back on 83 minutes before former Rams loanee Leon Best grabbed a brace, with the equaliser coming in stoppage time.
Wassall was understandably frustrated at seeing maximum points slip through Derby’s grasp after producing what he felt was some of the best football they had played all season.
He told Rams Player HD: “I’m flabbergasted and very, very disappointed.
“For 82 minutes, everybody could see that it was the best football that we had played all season.
“A game, in which we have played our best football, has only counted for a point, but what happened in the last 10 minutes was totally unacceptable.
“We absolutely dominated the game, played the best football we have done all season and what happened cannot just happen.
“We can’t be three goals up with 10 minutes left and draw 3-3, but we’ve done it.
“We need to start again on Monday, but that’s something that I can’t even think about at the moment because this was totally unacceptable.”
Wassall was even more disappointed to see Derby record only a point after dominating for the majority of the game.
The Head Coach had called for a reaction following the 2-0 defeat at Queens Park Rangers in midweek.
For 82 minutes Wassall got what he asked for but, ultimately, was forced to settle for just a point.
He said: “We asked for a reaction and for 82 minutes that’s the best football that we have played all season and they deservedly went three goals in front.
“It could have been a lot more though.
“We had so much possession, so much quality and created a lot of chances. That, though, means nothing if at 82 minutes you concede a goal and then completely collapse and draw the game.”
Wassall says Derby now face a test of character as they prepare for next weekend’s East Midlands derby at Nottingham Forest at the iPro Stadium.
The game is also the Rams’ final Championship fixture before the final international break of the campaign.
He said: “We will see how much character all of us have got. Rather than getting on the bus with a handsome victory, we feel like we have lost the game.
“We haven’t, we have drawn 3-3 and totally undeservedly, but you get what you deserve in football.
“For 82 minutes, we played the best football that we have done all season and the last 10 minutes was inexcusable. The way we capitulated after conceding a goal was totally unacceptable.”