Continuity mistake: When the princess and her chambermaid are talking about William and how they killed his wife, at the end of the conversation, the princess leans back against the edge of the archway. In the next shot from over her left shoulder, she's not leaning back, but sitting straight up.
Continuity mistake: After being knighted, William says that he will invade England. In the close-up shots of Robert in the group reacting to him saying this, you can see the man whose bride was taken away by the English lord earlier in the film behind him. But, in the wide shots, he's not there. It's just some random man behind him.
Audio problem: At the end, when the sword is thrown and sticks into the ground, the sound of the sword sticking into the ground is not synced up with the impact.
Character mistake: At the beginning, in the funeral of the young lady, the priest says in Latin the traditional "In nomine patris et filii et spiritus sancti", but he says the last word wrong, "sanctus" instead of "sancti."
Continuity mistake: After Wallace kills the noble and his crew for killing his wife there's blood on his face. At the funeral there is no blood on his face. The night of the funeral the blood on his face returns.
Chosen answer: Just like he said, it would dull his wits and not leave him fully lucid the next day when he had to face the torture.
LorgSkyegon
He wanted his full awareness so he didn't accidentally yell for mercy.