LastPass causing incorrect email address to be saved in new user form
After disabling my password manager, LastPass, my email address stopped sneaking into the username field. Upon closer inspection, here's what was happening:

After filling out the new user form and clicking 'Save,' you are prompted for your password. THIS is where LastPass misbehaves, or more accurately, does what it's designed to do but not very helpfully. It not only fills the password input, but replaces the contents of the 'Username' field with your email address (that is, the email address associated with the login stored in LastPass). It's easy to miss this, however, with the transluscent white overlay.


Solution

Configuring LastPass to not fill forms on the new user page isn't helpful, as you likely want it to enter your password when prompted. The best solution I could come up with is to clone my Craft login, delete the email address / username, and add 'New User' to the label. Now when prompted for the password I can select that login and avoid overwriting the new user's email address.