The earliest one I have up is from 2007, but I had a few before that. One of them, probably the first one, was of me performing the move X-Strike in the game Chrono Cross and doing the most damage I could. Back then I only used YouTube as a place to host videos of things that I wanted to be able to easily show others but for things that only a video could show correctly. It was like when someone creates a screenshot of their video stats and posts it here to show something they are asking about.
The earliest video I have up is showing a part of Chrono Cross where the dialog doesn't match the person who is talking. In this game, there are a lot of playable characters and you can have any character who has already joined you on your team at any time, so they did the dialog so that it could be said by whoever you currently have selected. But the part of the game is after one of your characters is supposed to have left your team permanently and they are talking about her. But I still had her on my team and she was talking about herself. The best thing is that it wasn't a cheat. The way the game worked made it fully possible to do so, but it was such a convoluted method that most people hadn't done it or thought of it. It wasn't so much a glitch as the game creators didn't expect anyone to do it when they were writing the dialog.