Bertrand Russell presents a scenario where one event seems to cause another, yet they are not contiguous at all. Russell's claim is supported by the idea of a causal chain, where the first event causes the last through a chain of events. Therefore, the cause is not contiguous to the effect. Also, Hume never defines priority very exactly either. If so, then the idea of priority is incorrect.
Constant conjunction presents some problems as well.
Hume himself throws out inductive reasoning as a form of sense experience, or his definition of causation would be much different. There is no way to predict the interaction of two events in the future, or the far past. Therefore, all events we see are merely presently conjoined, or conjoined up to this point in time, but not constantly conjoined. These questions and contradictions throw serious doubt on Hume's definition of causation. However, it is essential to remember that Hume's original point concerning what causation is not has been unrefuted so far.