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The tenth season episode "The One With Ross's Grant", featuring Greg Kinnear as Charlie's ex-boyfriend, Benjamin Hobart. Basically, Hobart is in charge of a committee to decide who gets the grant that Ross is applying for. He attempts to blackmail Ross into breaking up with Charlie (Ross calls him out on as being "crazy", but which he insists is "romantic"). When Ross tells Charlie about this and the two of them go to confront her ex, she agrees that it is romantic, and proceeds to kiss Hobart RIGHT IN FRONT OF ROSS! It's particularly bad since Charlie had been built up for several episodes as ideal for Ross. Hobart is said to have broken up with her (which he explains as due to fear of commitment to her); and it leaves Ross with no dignity whatsoever. It strikes one as a completely contrived incident for the sake of leaving Ross free to be conveniently paired with Rachel at the finale. Even if Ross couldn't do anything about Charlie, one wonders why he failed to report what was a clear violation of ethics on Hobart's part, especially after losing both the grant and his girlfriend. Had he reported this, Ross could possibly have held onto his grant, at least.
Monica's Flanderization of her neatness and bossiness hits its peak when acting as Phoebe's wedding planner in "The One With Phoebe's Wedding". She is incessantly rude to Phoebe (amongst other people) and cuts off the speeches at the rehearsal dinner midway through to fit her entirely arbitrary timetable. Phoebe, justifiably, fires Monica and decides to organise the wedding herself. Monica attempts to get her job back, but Phoebe rejects, knowing exactly what will happen, and Monica proceeds to delight in Phoebe's struggles. Hell of a way to treat one of your best friends on what's supposed to be the happiest day of her life.
'The One with the Shark', where Monica walks in on Chandler masturbating to porn; he jumps up and quickly changes the channel to a shark documentary. Despite seeing how he panicked and jumped, it never occurs to Monica that maybe he changed the channel; she honestly believes he gets off to sharks?!
The sheer On and off and on and off and on Ross and Rachel. Their shouting matches were initially quite upsetting to watch and very dramatic, especially the infamous one in "The One The Morning After" due to Ross having sex with the coffee shop woman in the previous episode, believing him and Rachel to be "on a break", which takes up a considerable amount of the second half of the episode and resembles a genuine argument. As the series go on, however, and the shouting-matches increase in frequency from season to season, the viewer becomes more and more compelled to bash their heads together and tell them in no uncertain terms to shut the ''hell'' up.
Phoebe manages to out-crazy herself and believes that her dead mother returned as a cat. She refuses to return the cat to its rightful owner, an eight year old little girl. When Ross calls her out on this, she gets mad at him for being "intolerant" of her beliefs! The other characters, while they secretly agree with him, do not support him. And, as if Ross wasn't being humiliated enough, he is forced to apologize to the cat.
In the commentary for this episode, it's revealed that producer Marta Kaufmann was the one who pushed for that subplot after her own mother passed away, which explains the akwardness of it.
It's also been said that others on the writing staff would've normally nixed anything like this, but given the circumstances, didn't have the heart to.
In "The One With the Birth Mother", Chandler and Monica are mistaken for a doctor and a reverend on some adoption forms when she meets the potential mother of her child. Rather than deciding to correct the mistake, like a sane person would, Monica goes into full-blown Jerk Ass mode and decides to fake being a reverend just so she can get the woman's baby. In the end, Chandler brings Monica around and talks the woman into giving them the baby (showing surprising compassion considering his own character) after she understandably storms out on them for lying, but come on.
In "The One Where Heckles Dies", Phoebe repeatedly and consistently denies the Theory of Evolution to Ross, who is a doctor of paleontology and a full-time paleontologist. Ross even brings in fossils to prove it to her, instead Phoebe breaks him down and makes him doubt the theory. As a doctor. Of paleontology. A man that spent several years studying fossils to become a doctor. Of paleonotology. Now, Phoebe is an oddball cloudcuckoolander and can be forgiven for not believing in evolution (being as you'd learn it in most tenth-grade science classes and Phoebe never went to school), but to actually deny it, thereby denying Ross's entire work, and disallow him from proving it to her, revealing she said she didn't believe just to get a kick out of making him doubt himself, and then gleefully forget all about it? That's pretty damn cold, Pheebs!