This is an entry for the 16th Fortnightly Challenge.
You won't believe what happened at work today! I was working on a new software when Robert, the colleague I work with on this, got mad in a second : he banged his table, shouted the f-word and left the office! Well, one thing I know is that he was not really working on the software... All the morning, I've seen him writing things on pieces of papers. That's why once he had left, I went to his desk to understand his rage. Before he came back, I was able to take a glimpse at this piece of paper.
At first, I was... puzzled. But then I remembered where I had seen this grid before. I smiled.
And you, can you find out why Robert left in anger?