Maybe y'all already knew this one, but I didn't.
Have you ever had a FASTA sequence that software just wouldn't recognize as a FASTA? Like, you saved it as "XMAn.FASTA" and you knew that the sequences inside were fine, but you just couldn't open it?
Well, it turns out that Windows has this fantastic little default feature called "Hide Extensions for Known File Types" (Sorry, you'll have to click on the picture to see what I'm talking about)
In this case my copy of Windows 7 Pro identified my .FASTA as a simple .TXT file and to clean it up it went ahead and added a .txt to the end of my file...then hid the .TXT.
To find this feature and turn it off so it never makes you consider smashing your keyboard late at night ever again, simply go to the search bar and type "Folder Options" then turn off this idiotic option and hit Apply. Then you can go to your folder and delete the stupid .TXT and everything should be working again!