You could say I'm partially (or entirely?) to blame for this for not having done enough research beforehand. However...
A while back I wondered if there were a math-related site on the StackExchange network. So I typed mathematics.stackexchange.com into my browser's address bar and was promptly redirected to Math Overflow.
I soon discovered that I do not really belong on that website due to its graduate/research-level nature.
This was really not such a big deal; but the URL redirect that initially led me to MO in the first place had now led me to believe that MO was the math-related SE site.
Only some time later did I learn that, in fact, math.stackexchange.com is the general-purpose (not graduate/research-level only) SE site for math.
Given the similarity in the URLs, it's quite conceivable to me that I may have never discovered that this site (math.stackexchange.com) even existed.
Is there any way that navigating to either mathematics.stackexchange.com or math.stackexchange.com could notify the user of the other site's existence, with a brief 1- or 2-sentence explanation of the difference between them?
I realize MO is not a part of the SE network, so this would likely require coordination with them. Still, it seems worth doing (to me), to avoid potential/inevitable confusion or even, in the worst case, the loss of a potential user.