Listen, we’re not scientists. A slightly brown edge is not the end of the world. Cut it off. Eat the rest.
But here’s a hard rule: If any part of the head has slime, mold, or a truly awful smell, do not negotiate. That’s like trying to burn a candle that’s already melted into a puddle. It’s not happening.
Old lettuce can also develop a bitter taste. Even if it looks fine, if it tastes like regret, don’t force it. Save yourself.