The filter term "contains" can catch irrelevant emails because it matches substrings, not just entire words. The user may not expect this behavior. It can create a significant number of unwanted matches, especially when applied to message bodies.
I suggest adding a new filter term "contains word(s)" which only matches entire word(s).
Examples: My filter term "contains" "tyc" caught emails containing terms like "http://www.marriott.com/reservation/availabilityCalendar.mi" (look for it), and in random strings like generated links.
(For the curious - "tyc" is the abbreviation for Tiburon Yacht Club.)