A halberd does triple damage on a critical hit and double damage when set to receive charge.
What happens when you set to receive charge and you get a critical? Is it double or triple damage?
The Glossary on Multiplying says
When you are asked to apply more than one multiplier to a roll, the multipliers are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you are asked to apply a ×2 multiplier twice, the result would be ×3, not ×4.
A halberd's critical hit multiplier is ×3 and it deals ×2 damage when braced for a charge so a critical hit when its braced for a charge deals ×4 damage.