I was searching for measure-theory questions which had the word "algebra" in them.
At first I was searching "algebra [measure-theory]" and much to my surprise the result was that it searched for "[algebra] [measure-theory]" instead.
I did a couple of tryouts and it appears that the search automatically assumes the first words are tags, unless encapsulated with quotes.
If this is by design, then this is a bad design in my opinion. If this is not by design, well... it is a bug.