Having only just finished this year, The Big Bang Theory was an extremely popular and long-running sitcom that followed 4 geeky scientists and their neighbor, Penny, as they navigated through society's expectations. Each core character had their own special quirks but the one thing they all had in common was that they were clearly the best of friends.
Despite their remarkable friendship, their personalities also put them at odds with each other fairly frequently, none more so than Sheldon and Leonard. These two loved each other, but Sheldon's obnoxious obliviousness and Leonard's selfishness meant that they did do some pretty horrible things to the other.
Sheldon: Stole The Credit For His And Leonard's Research
Unsurprisingly, the first entry on this list is about Sheldon screwing over Leonard. Part of Sheldon's hilarity stemmed from the fact that he honestly doesn't register when he's being rude or selfish. Unfortunately, this also means that he treated his friends like dirt on multiple occasions.
In the show's eighth season, he and Leonard co-wrote a scientific paper which proved to be quite popular with the scientific community. However, Leonard felt betrayed when a magazine covering the paper neglected to give him credit. This was all down to Sheldon who deliberately failed to correct the mistake.
Leonard: Doctored Sheldon's Experiment Results
Way back in Season 3, Leonard, Howard and Raj had agreed to accompany Sheldon to the North Pole for 3 months to assist him with an experiment. Apart from Sheldon, none of the gang were particularly looking forward to spending time with Sheldon for that long in an enclosed space.
Indeed, Sheldon apparently got very annoying very quickly, which was only exacerbated by the fact that his experiment was failing to produce any desirable data. To prevent themselves from killing their friend, Leonard persuaded Howard and Raj to help him forge new results in order to pacify Sheldon.
Sheldon: Constantly Belittled Leonard
If you haven't realised by now, Sheldon was a particularly difficult person to like and get along with. This makes the fact that Leonard remained his roommate for 8 plus seasons quite an impressive feat of endurance. During that time though, Sheldon definitely tested the boundaries of Leonard's patience tenfold.
In his own way, Sheldon always looked out for his roommate and best friend, but he also couldn't help but criticize everything that Leonard did. His favorite topics to mock were Leonard's work, his education, his relationships... Basically anything that Leonard did, Sheldon disparaged.
Leonard: Called Sheldon's Mom When He And Penny Were Fighting
After 12 seasons of The Big Bang Theory, Sheldon and Penny's beautiful friendship is probably the purest thing to come out of the show. These two were always there for each other and Sheldon confided in Penny things he wouldn't tell anyone else, not even Amy. However, they didn't start out that way.
In Season 2, Penny was still getting used to Sheldon's mannerisms, and Sheldon still didn't view Penny as part of the gang. This led to one of their biggest, and most hilarious, fights of the series. It only ended when a fed up Leonard gave Penny Sheldon's mom's number. No matter how annoying they're being, you don't drop your best friend in it with their mother.
Sheldon: Acted As A Surrogate Son To Leonard's Mom
Moms were clearly a big issue when it came to Leonard and Sheldon. Leonard in particular had it bad, as his mother constantly maintained a cold and unfeeling detachment from her children. As a result, Leonard never felt that close to his mom and frequently tried to earn her approval whenever she visited.
To his dismay, he learned pretty quickly that his mom could love and praise people if she so chose. Unfortunately, that person was Sheldon. Oblivious to Leonard's hurt and jealousy, Sheldon continued to bond with his roommate's mother, who heaped on praise for his work and emotionless way of living.
Leonard: Presented Shared Research Without Sheldon
Back in Season 1, the audience was only just getting used to Sheldon's eccentric behavior. What they weren't prepared for however, was just how quite unreasonable he could be. One of Sheldon and Leonard's research papers was selected to be presented at an academic conference. The only problem was, Sheldon refused to go.
He considered it beneath him which severely disappointed Leonard, who simply resolved to present their research by himself. In fairness, Sheldon was being a complete and utter jerk, but by presenting their research on his own, Leonard was basically taking most of the credit, which wasn't fair to Sheldon.
Sheldon: Interference With Leonard's Relationships
It's difficult to narrow down just one instance of Sheldon messing with Leonard's relationships, so we've lumped all of them in together. Sheldon's complete obliviousness to, and disgust for, anything sexual is the main factor here, but it's hard not to feel sorry for Leonard and his girlfriends.
Sheldon has an unfortunate habit of visiting Leonard's room late at night, meaning he has interrupted Leonard's various sexual encounters surprisingly frequently. He also made Leonard come across as clingy when the latter was dating Stephanie. Sheldon even tried to condition Penny to be less irritating when she and Leonard were dating for the first time.
Leonard: Broke The Roommate Agreement
The Roommate Agreement and its various counterparts were a hilarious and long-running gag on The Big Bang Theory during its run. The Agreement was basically a way for Sheldon to get his friends to do whatever he wanted without them arguing back.
However, the Roommate Agreement was frequently broken by both parties. Whenever Sheldon was challenged, he would conveniently find a loophole that enabled him to have his way. However, Leonard was worse. In Season 4, he had his lawyer girlfriend, Priya, look over the document and argue his case, which temporarily stunned Sheldon. Not cool dude.
Sheldon: Ignoring Leonard And Penny's Pregnancy
To be fair to Sheldon, he has always made it quite clear that he thinks the world should revolve around him, and he has never attempted to hide that fact. So, we shouldn't have been surprised that when Leonard and Penny told him that they were expecting, he treated the news as no big deal and made it about himself.
However, this time, Sheldon had gone too far. Leonard had always wanted children and a large chunk of Season 12 had dedicated itself to reconciling that with Penny's desire to remain childless. Basically, this pregnancy was amazing news for Leonard and Sheldon simply couldn't care less.
Leonard: Came Home Early But Didn't Tell Sheldon
In the Season 6 finale, Leonard was offered the chance to go on an expedition to the North Sea. Despite having misgivings, Sheldon allowed him to go with relative good grace. In the Season 7 premiere, Leonard was still away at sea so Sheldon and Penny were keeping each other company while obviously missing Leonard.
Leonard then came back in the following episode to surprise Penny and spend a few days (and nights) with her alone without any interruptions. It would have been a sweet gesture, had it not been for the fact that the audience watched Sheldon struggle with Leonard's absence during the whole of the previous episode. Leonard neglecting to let Sheldon know he was back seemed a bit callous and selfish.