Assignment Bias:
Imagine researchers comparing educational programs, and they decide to use two schools as their participants. They find that there is a difference between the two groups and conclude that treatment A is better than treatment B.
However, they neglected to take into account the fact that the schools contain children from different socio-economic backgrounds, and this created assignment bias. A better idea would have been to use children from a single school or use random assignment, but this is not always possible.