I think this is probably the instructable that brought me to the site all those years ago. I wanted to build one of these (which I still haven't done) and was looking up amplifier circuits using Google to understand it better.

I found this in the search results and then started exploring the site. I would have been nearly 13 at the time, going by the date this was published and the date I signed up for an account. There were fewer than 800 instructables on the site in total back then. I registered as soon as I was old enough (i.e. on my 13th birthday). (I also created a Wikipedia account the same day.)

Another of the search results was a blog post. I commented on it. Back in those days, blog comment systems commonly allowed you to use a few HTML tags to format your comments. I put a link in my comment, but forgot to close the <a> tag. After I posted the comment, the entire page from the link in my comment to the bottom was a link! I couldn't use the comment form to post another comment containing </a> to fix the problem (or notify the blog's owner), because as soon as I clicked in the comment form, I went to the page I'd linked to! (I don't remember if I tried tabbing to the form. Also, it was probably partly my browser's fault, because it should have auto-closed the unclosed <a> tag within its containing element rather than letting it affect the rest of the page.)