The documentation for the overflow states:
OVERFLOW – Indicates that events might have been lost or discarded.
It does not say under what circumstance should I expect event to be lost or discarded? At first I thought it would be a result of writing a lot of files very fast to the folder. I created a few thousand files with zero size and moved them to a monitored directory. No OVERFLOW.
What am I missing?
"File systems may report events faster than they can be retrieved or processed and an implementation may impose an unspecified limit on the number of events that it may accumulate. Where an implementation knowingly discards events then it arranges for the key's pollEvents method to return an element with an event type of OVERFLOW. This event can be used by the consumer as a trigger to re-examine the state of the object."
From JavaDoc.
See also Steven C's answer. His point about unconsumed events makes the difference, I think.