The first Teen Girl Squad episode is the only one where the "IT'S OVER!!!" ending card is actually written on top of the last scene rather than on a separate card.
The first Halloween Episode of Homestar Runner (the website the above spun off from) is the only one in which Homsar does not appear (it was released before the second Strong Bad Email ever, which marked the character's debut).
The Strong Bad Emails used to run shorter, often consisting simply of Strong Bad reading the email and writing his reply. The more recent ones usually include extended fantasy sequences, flashbacks, interruptions by other characters, and Strong Bad walking away from his computer to do other things which may or may not be related.
Heck, look at the first few cartoons. "Marshmallow's Last Stand" has some pretty odd animation and character designs, and most of the characters themselves behave differently than in later videos.
Also, Puppy wouldn't develop his sex-crazed personality until Episode 2. He wasn't even in the part of Episode 1 with the naked chick.
The pilot of Happy Tree Friends from way back in 1999. None of the regular characters appear, though the characters in the pilot do somewhat resemble some of the ones from the rest of the series; and the episode was less than a minute long. The opening and end sequences were also completely different from all later episodes. Also, the head of a decapitated character bites someone, which would never happen in the current show.
Neurotically Yours in the first handful of episodes was very rough and different compared to the rest. The animation quality was very rough, Germaine was a Flat Character who didn't speak, Foamy spoke very fast and said whatever was on his mind, and was more of an actual humor-based cartoon with original jokes and scenarios, compared to the more sociological rants seen today.
The Madness Combat series didn't start out so mad, or combative for that matter. The first in the series was about a silly cannon ride that shot people into a giant marshmallow. There were a few deaths in it, but all of them were by accident. The second was an unnamed protagonist just trying to dance the "Chicken Dance," and most of the violence was fisticuffs, though the seeds of the ultra-violent gunplay were sewn there.
The first five episodes of The Frollo Show were written as YouTube Poop rather than coherent narratives. The first five episodes were re-cut into a single video, eliminating the nonsensical scenes and only keeping scenes important to the story. Among the weird differences of the early videos include:
In her initial appearance in the first episode, Frollo wanted to rape Madotsuki. In later episodes, she is a valuable ally to Frollo, and played a key role in launching the events of the second saga, "Frollo Finally Does It".
A lot of Frollo's sentence-mixed dialogue was hard to understand and had to be subtitled. Recent episodes replace this with sound clips of Megabyte from ReBoot, who was also voiced by Tony Jay; both performances sound identical, allowing seamless blending.
In the first two trailers that introduced RWBY, both Ruby and Weiss only showed small signs of emotion, leading many viewers to conclude that they were both Emotionless Girls. Once the series started, it was very clearly established that neither of them were very emotionless at all; Ruby is a slightly oddGenki Girl and while Weiss is a Defrosting Ice Queen, she's still very expressive. Word of God was that the first trailer was more of a "weapon resume" that was meant to demonstrate Ruby's fighting abilities, not her character. In contrast, Blake and Yang have more or less the same characterizations in their introductory trailers as they do in the series.