Selection bias. In research using comparison groups, some participants in one group may be different from those in the other group(s) in ways that affect performance. For instance, an experimental group may do much better on a post-test than the control group, not because the experimental intervention was effective but because more of the experimental group members figured out how to take the test (See number 3 above.). Similarly, the pre-test/post-test differences between the experimental and control group may be small, suggesting that the intervention did not work. However, in fact, the control group contained many people who WERE likely to change as a result of maturation or some historical factor, and so they gotr high scores even though they received no intervention.
This source of invalidity can be handled, in part, by random allocation of participants to comparison groups. This way, all possibly biasing factors have an equal chance of being in both groups.