Sign 'at sign' does not working properly on Czech Keyboard. It's not fault of keyboard i don't have this problem alone. Also my friends complaining on this bug. It's pretty annoying, when I must switch between english and czech keyboard to write selector in command block.
In 1.7.10-, it's working without problems, but now you must spend few minutes with hard presing 'Alt Gr + V' to get only one -at sign.

How to reproduce:
1) Set input language to Czech.
2) In minecraft open the command block or chat.
3) Press Alt Gr and V.
4) And now you can press that buttons to your death.

Otherwise you can use 'ctrl + alt + v' to write -at, but now it's just pasting copied text