Iron Golems given a Strength or a Weakness potion are not buffed/debuffed in strength, respectively.

Steps to Reproduce:
1. Make a new Creative world.
2. Create two Iron Golems at a distance from one another.
3. Give one a Strength II potion and the other a Weakness potion.
4. Spawn Skeletons near the Iron Golem with Strength II (make sure it's night)
5. Observe how it takes usually 1-2 hits to kill the Skeletons, which is how it is without the Strength potion.
6. Spawn Skeletons near the Weakness Iron Golem and notice the same results.

I believe this bug is due to the fact that Iron Golems deal different damage for each hit instead of a constant damage like everything else.