What is most likely to occur when a calcium supplement and an iron supplement are taken simultaneously?

Decrease absorption: In low dose, nothing probably will happen. Calcium and iron are taken together in a multivitamin all the time. At higher dose, you might get GI upset. Regarding efficacy, you get less iron absorption if taken with calcium. It is recommended to take iron 2 hr before or 4 hr after calcium supplementation.

Poor iron absorption: The concern is that the calcium salt can combine in the stomach/ intestine with the iron in a way that makes the iron less available for absorption. So separating them in time by hours will avoid the issue mostly.