There is one hex in the very bottom right hand corner but your screen seems to be cutting it off. Do you remember what resolution you were playing at?

In the meantime you should be able to play the level using a wider resolution in windowed mode to be able to see the last hex.

I'm playing in window mode 1400x1050, running Kubuntu 13.10 64-bit.
I see the missing cell in fullscreen or when I drag the window a little wider.

Hello! I'm bumping this thread because I just ran into the same problem. I'm playing full screen on a 1280x1024 monitor, running debian. I completed the level by re-opening the game in 16:10, but I really wondered what was going on at first. Just mentioning this in case you'd like to fix it.

With a 4:3 ratio, the first few characters of all tutorials are also hidden.

EDIT: Level 6-4 is also unplayable with that ratio.

Confirmed missing tile in 1600x1200 resolution, also in windowed mode when expanded fully.