Lehmann was filthy that Siddle had cost the side a crucial wicket with a front-foot no ball.
Instead, replays showed he'd overstepped, and England piled on another 100 runs before the fifth wicket fell.
It's not the first time Siddle and the Australian quicks as a whole have let themselves and the team down by committing the cardinal sin of bowling.
At the Gabba last year, Siddle should have had Jaques Kallis out for 43, only for the South African to walk off with 147.
QUICK SINGLE: Lord's Day One Wrap
Already firm underdogs to steal back the urn, Lehmann knows Australia can't afford to shoot themselves in the foot - especially with such a fragile batting line-up to come.
"There is no excuse. The line is there for a reason and it's unacceptable," said quick Ryan Harris.
"It cost us a lot of runs today and, potentially, it could cost us the Ashes. It may have happened last Ashes too.
"We were pretty disappointed. Darren was not very happy when we went in for tea."
Harris with 3-43 after a year out of the Test team, and Shane Watson (1-41) with his immediate impact were positives for Australia.
But it was part-time spinner Steve Smith (3-18) who changed the game.
Agar's debut was always going to be a tough act to follow, but the teenager had a forgettable day with the ball and also picked up a minor left hip strain when he chased a ball to the boundary.
Physio Alex Kountouris says Agar should be fine to bat and bowl in the second innings.
He'll have plenty to work on, after learning some harsh lessons about what can happen when you bowl flat in Test cricket.
However, Pattinson was the biggest disappointment.
Billed as the spearhead of the attack, the Victorian seemed to struggle with the slope and occasion at Lord's.
Pattinson was able to shape the ball early, but could never get his line right.
He'll need to fight back hard in the second innings, with Mitchell Starc and Jackson Bird knocking at the door for recalls for the third Test.
"I don't think he had his greatest day but I still thought he bowled ok in patches," said Harris.
"I don't know if it was the slope.
"I'm sure he'll bounce back in the morning."
The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of Cricket Australia.