I'm a relatively new user of Mathematica, and the following behavior seems odd to me:
In:= _ + _ Out= 2 _
Use of `Plus` here is just to illustrate a more general "problem with `Blank`. I had expected pattern objects to resist such evaluation. This is possibly related to the fact that I was surprised by the following:
In:= Blank == Blank Out= True
Since each Blank can match anything, I find this conceptually to be the wrong behavior. As an example of a counter-intuitive result:
In:= MatchQ[a + b, _ + _] Out= False
I understand that I can deal with this problem by using HoldPattern.