The threat message from Charlie that Walden reads on his phone is an actual real life tweet of Charlie Sheen about Ashton Kutcher becoming the new star of the show.
Charlie Sheen made bizarre statements in television interviews, suggesting that he was a "warlock" with "tiger blood" and "Adonis DNA," and that he was "winning." These things were repeated as threatening texts from Charlie that Ashton Kutcher (Walden Schmidt) gets from him.
Lieutenant Wagner asks if Charlie considered anger management, to which Alan replies "yeah, but it didn't work," which is a reference to the show Anger Management (2012), which was the show Charlie Sheen was in after he was fired from this one.