cum hoc ergo propter hoc

The fallacy is to assert that because two events occur together, they must be causally related. It's a fallacy because it ignores other factors that may be the cause(s) of the events.

Latin for "with this, therefore because of this". Slightly different than post hoc, ergo propter hoc, it describes the logical fallacy that because two events occur at the same time, they must be causally related--for instance, changing lanes with my car causes the rest of the traffic to speed up or slow down.