Did both Mr. Beers worth of beer sour? Or only 1? If they both did, there's likely somthing wrong with your sanitation regimen. If only one did, you would have to think that it was an issue with that Mr. Beer kit alone: not cleaned properly, or not sanitized well enough.
If you have moved on to proper fermentation vessels, then the second scenario is moot. If they both did sour, then we need to look at your sanitation. Anything that touches the wort post-boil needs to be dunked/sprayed off with a no-rinse (preferably) sanitzer like StarSan or Iodaphor. As a safety measure, you might even want to bleach bomb your equipment, making sure to rinse it really well.
If this is all your normal MO, then we'll have to look elsewhere. Do you crush your own grains? If so, are you doing it in the same area as the reast of your brewing? Bulk grains carry a plethora of nasties, and milling them can make the bugs airborne. When the dust settles into your wort, you're just asking for infections. Make sure your brewing area is relatively free of dust and grime.