Mischa Barton Filming Scenes for Law & Order

Mischa Barton badly bombed during filming of "Law & Order: SVU" on Monday, according to an insider - and more of her infamous hard partying may be to blame.

After reports surfaced yesterday about the troubled starlet continually forgetting lines while filming a scene with Chris Meloni and Mariska Hargitay - she played a hooker in her guest spot - our source acknowledged that Mischa's been rocking out all over NYC.

"She's a regular at Antik," the snitch says of the downtown hot spot. "She usually comes late every Thursday and leaves late. A lot of times she'll even go to another club after she leaves."

While that may seem like a normal night for many a 23-year-old, it might not be the best idea for Barton: The actress has a history of big-time partying that came hand-in-hand with substance abuse.

In July, she was admitted to Cedars-Sinai Medical Center on involuntary psychiatric hold after reportedly having a drug- and alcohol-fueled meltdown in her L.A. apartment.

She had an earlier stint in rehab after being arrested for DUI in December 2007.

A rep for Barton said, "Everything's going swimmingly on set," but our insider insists that's not the case.

"She's been 'off' at work, and I'm not surprised, judging from how hard she parties," our source says. "It's scary to think what will happen if she keeps going out like she does."

Perhaps the wayward starlet should have been home on Thursday night studying her lines (she apparently had only seven). After all, her gig on "SVU" was going to be the comeback she needed.

Ever since her show "The Beautiful Life" was canceled by CW after just two episodes, Barton hasn't had much work.

"People close to her were hoping this would get her back on track," the source says. "But it looks like she's going to need yet another chance."