Accounting for Lawyers - S2-E2
Character mistake: At the party, Jeff is talking to Ted about his parents' divorce, which included a custody hearing. Jeff mentions the divorce lawyer drove a Mercedes SLK. However, the SLK didn't come out until 1996, which would make Jeff at least 21 (turning 22) at the time of the divorce (he was born Nov 1974). There would have been no need for a custody hearing (Jeff was an only child between his mom and dad) and his parents divorced when he was very young, so the divorce had to happen prior to 1996.
Cooperative Calligraphy - S2-E8
Character mistake: Right before Jeff searches Abed's bag, Britta explains to the group that Abed is the innocent one. After Jeff reminds her that Abed put gum in her hair, she looks at no-one in particular and says "empty the bag, Abed." Jeff is the only one holding the bag, with Abed standing five feet away from both of them. It's clear this line was directed to Jeff, but the wrong name was used.
Intro to Political Science - S2-E17
Character mistake: When Troy and Abed are covering the school election, on Jeff's detail page, kryptonite is spelled "kyptonite." (00:07:15)
Custody Law and Eastern European Diplomacy - S2-E18
Character mistake: Annie when giving her gifts is holding two little mini flags to cover the possible ethnicity of Shirley's baby. One is a Chinese flag and the other is Zimbabwe. Annie makes reference to the fact that the ethnic food she made is one from Ghana. She makes Ghana ethnic food but does not get the right flag for Ghana. She decorates the table with numerous flags which are also Zimbabwe.
Foosball and Nocturnal Vigilantism - S3-E9
Character mistake: In the bonus clip where we see Leonard's online video about frozen food, the first thing he says is, "This is November 29th". However, if you look under the video, you can see the video was uploaded on Nov 26, 2011, which is three days earlier.
Answer: His attempt to "kill" Jeff was to roll at him as an iPad on a stick. He knew it wouldn't work, but it played to the story that he was a killer as he knew Jeff would tell people about it, or attempt to retaliate for it.