In Minecraft 1.6.4 you are able to screenshot the chat when the chat window is open. I believe that this supposed to be the case in 1.7.2 as well, but it isn't.

I opened 1.7.2, logged into my new server, talked to my friend, and I hit the F2 button to screenshot the chat, but nothing happened. I hit F2 again, but nothing happened.
I then closed the chat, hit F2, and the screenshot was taken (Of course I wasn't able to capture the text from the chat though).
I tested this on 1.6.4 and the screenshot was taken when the chat was open. I also tested this on both my server and single player in both 1.6.4 and 1.7.2.
I found that screenshots were unable to be taken in version 1.7.2 while the chat is open (1.6.4 was just fine)

Edit I found out just now that screenshots in 1.7.2 cannot be taken when viewing the contents of a chest as well. End of Edit

Steps to Reproduce:
1. Open Minecraft Version 1.7.2 through the minecraft launcher
2. Log onto any single player world, or any server
3. Hit "t" to open the chat
4. Type something, and keeping the chat open, hit F2 to screenshot
5. Screenshot will not work until the chat is closed.