[timberburn]

When I was setting up an account on Comcast's website, I was consistently getting an nondescript internal server error when submitting the form.

Took me quite awhile and many failed attempts to find that Comcast will throw an error when your requested username contains "comcast".

[josegonzalez]

Same for ConEd.

Moneygram will freeze payments with the word "moneygram" in the associated email. Which is great for those of us that use catch-all emails and use the email address to discern what to do with an email...

[rincebrain]

I've had a number of fun failure modes like that.

My current favorite two include when the change password form permitted longer passwords than the login page, and one where the change password form happily allowed special characters, but if there was e.g. a semicolon in the password, submitting it from the login page would throw a SQL error.

[hidroto]

i wonder if that is to stop people from using names like comcastSucks or worse.

[rdancer]

It may not have been the rationale, but it sure must be the most common use-case.

[akurtzhs]

It also avoids security problems with fraudsters creating official looking emails - "comcastsupport" and the like.