The effect is caused by a number of objects or events, of which the cause identified is only a part. A variation of this is the feedback loop where the effect is itself a part of the cause.

Examples

1. The accident was caused by the poor location of the bush. (True, but it wouldn't have occurred had the driver not been drunk and the pedestrian not been jaywalking.)
2. The Challenger explosion was caused by the cold weather. (True, however, it would not have occurred had the O-rings been properly constructed.)
3. People are in fear because of increased crime. (True, but this has lead people to break the law as a consequence of their fear, which increases crime even more.)
4. "[A] great classicist, J.B. Bury ... eliminated depopulation, the Christian religion, and the fiscal system as causes of the dismemberment of the Roman empire. 'If these or any of them were responsible for [the Empire's] dismemberment by the barbarians in the West, it may be asked how it was that in the East, where the same causes operated, the Empire survived much longer intact and united.' But this is a mistake. The three causal elements which Bury rejects may have interacted with each other, and with still other elements, in such a way as to produce very different results in the West and the East." (Fischer, Historians' Fallacies, p. 179.)
5. "A necessary condition of an event is one that must be present in order for the event to occur; without the necessary condition, the event cannot occur. On the other hand, an event will occur in the presence of a sufficient condition. Confusing necessary and sufficient conditions may lead to the erroneous belief that the presence of a necessary condition is all that is needed for a condition to occur. Some explanations of the the cause of stuttering, for example, suggest that a genetic predisposition to stutter is a necessary condition for the disorder to develop. The genetic predisposition, however, may not be sufficient; it may be that one or more other conditions are also required. If that is the case, then not every individual with a genetic or constitutional predisposition to stutter will stutter, if the other conditions are not present. On the other hand, if the predisposition is necessary, then no one who does not have it will stutter, despite the presence of those other conditions." (Packman & Attanasio, Theoretical Issues in Stuttering, p. 14.)

Proof

Show that all of the causes, and not just the one mentioned, are required to produce the effect.