This has happened to me twice now.
The first time, I received a thick, unmarked letter from Columbus, OH. Of course I thought it was from Ohio State. It was my credit card company reissuing me a new card before the original expired.
The second time was more of my own fault. I got an email from Indiana State University. It was one of those you-took-the-GRE-now-here-is-a-bunch-of-spam emails. However, since my email window was squished, I only saw "Indiana" in the subject line, assumed it was from Indiana University, and proceeded to ignore everything else but the body of the message which told me that based on my credentials (GRE scores), they would love to have me in their department. It wasn't until the final line that I read something along the lines of "Please consider applying to our program" that I was tipped off that it wasn't from IU. Sad day.