assert [ 3, 4 ].every{ element -> element > 2 }  //true
assert [ fingerCount: 10, toeCount: 10 ].every{ key, value -> value == 10 } //true

Given that, you might reasonably expect that the every() method would return false when executed against an empty collection. But it doesn't:

assert [].every{ element -> element > 2 }  //true
assert [ : ].every{ key, value -> value == 10 }  //true 

There is a JIRA ticket about this issue: https://issues.apache.org/jira/browse/GROOVY-7207. The initial commenter's guess is probably correct: that the every() method starts with a default return value of true and only becomes false if and when an item in the collection fails the conditional test, so an empty collection fails to affect the default value of true.

Still, it's not what I would have expected, and I was unaware of the behavior until I encountered it for myself.