A Phrase "An unexpected happening came"

[SkyRunner]

Hello

I really want to know a phrase-idiom-proverb whit this meaning :

When something unusual happened ( not a bad thing )
but a thing that you never expected .

e.g. Your friend always come late but one time he came on time .
and sometimes with a sense of humour

[Yankee]

Hi Skyrunner

The first idiom that comes to my mind is 'out of the blue', but that isn't humorous. It just means 'suddenly and unexpectedly'.

"... but one time he showed up on time out of the blue.
.

[Mister Micawber]

.
Here's another:

My friend always comes late but one time he came on time-- you could have knocked me down with a feather!
.
_________________
Native English teacher at Mister Micawber's