This almost happened to me a couple of weeks ago at DFW. I checked aa.com at home before leaving for the airport; upon arriving at Terminal D (where my flight had just 20 minutes prior been scheduled to leave), I learned that it had been changed to Terminal A.

Not only did I have to run to make my flight... I had to check my bag at a different terminal (which at DFW seems to result in delayed baggage at times-- but not this time, thankfully).