Question

During your experiment, you are drive water from the hydrate MxXy*zH2O. You subsequently determine that your hydrate has six molecules of waters associated with each formula unit. However, the bottle the hydrate came from indicated a formula with only four molecules of water. What went "wrong" in your experiment?

All I can think of is the bottle could be mismarked.

Answer 1

That is real confidence in your lab skills to suggest it couldn't be your fault. But I think the spirit of the question is for you to suggest what YOU did wrong, as well.
a. Heating the sample drove off all the water but also decomposed the sample, making the weight loss too high.
b. The bottle was mislabeled (your original answer).
These two may help get you started.

Answer 2

I think the answer is that the bottle of compounds were previously opened, thus altering the water content.