I like the word quirk when you want to convey the notion of something that's a little unexpected but not overtly unwelcome (like an error or a failure).

Definition: http://www.merriam-webster.com/dictionary/quirk

Full Definition of quirk

 

b : a peculiar trait : idiosyncrasy c : accident, vagary

Consider also the word vagary, listed there as a synonym to quirk.

I like the word quirk when you want to convey the notion of something that's a little unexpected but not overtly unwelcome (like an error or a failure).

Definition: http://www.merriam-webster.com/dictionary/quirk

Full Definition of quirk

b : a peculiar trait : idiosyncrasy c : accident, vagary

Consider also the word vagary, listed there as a synonym to quirk.