Illusory correlation
Illusory correlation is perceiving a link between two things that are not actually related, or seeing a stronger link than really exists. It often happens because the times the two things occur together stick in memory while the other times are forgotten.
In everyday life
A nurse believes the emergency room gets busier during a full moon, because busy full-moon nights stand out while busy nights under other moons are forgotten.
You are certain your knee aches more before it rains, because you remember the sore days that were followed by storms and forget all the rest.
Why your mind does this
Our ancestors survived by spotting patterns fast: rustling grass might mean a predator, and dark clouds meant rain. Filling gaps with a plausible story lets us act on thin information instead of freezing. A brain that sometimes sees a pattern that isn't there loses less than one that misses a real threat.
Its family, in 30 seconds: Stories & patterns
How to spot it
Ask whether you have also counted the times the two things did not happen together.
How to counter it
Make a simple four-box tally (both, only the first, only the second, neither) before believing two things go together.
Same family: Stories & patterns
SourcesChapman (1967)·Wikipedia