The phrase "unusual occurrence" refers to something that is not common or ordinary. It represents an event or situation that is out of the ordinary or unexpected. Full definition
And if the squad are still talking about the one time he did speak out and it was two years ago, then it must be a very unusual occurrence. (justarsenal.com)
In what can be described as a very unusual occurrence, the trial proceeded without any medical opinion evidence addressing the extent of injury. (bc-injury-law.com)
The exchange attracted much attention when investigators smelled unusual occurrences in bank accounts which were stemming from 300 sim cards and bank cards which are usually used for storing the digital currencies. (kryptomoney.com)