How not to win friends and influence people as post-rehabbed Mischa Barton‘s "unprofessionalism" stinks up shooting of Law &Order SVU.

According to RadarOnline.com, Mischa spent all day Tuesday flubbing her lines and the show only shot one scene the whole day – which rapidly became a 10 hour day because of Barton’s boo-boos.

A production insider divulged that star Mariska Hargitay and the cast and crew on set were losing patience with Mischa’s "unprofessionalism."

"She only had seven lines, seven!" the source snapped.

 "Each time she would mess up she would forget her lines and Mariska kept prompting her and saying ‘it’s your line Mischa’."

A steady torrent of Mischa mumbling ‘"sorry" quelled some of the festering on-set unrest.

After a stint in an involuntary psychiatric hold during the summer, Barton bounced back to star on Ashton Kutcher produced The Beautiful Life which the CW quickly shelved after airing two eps.