Yeah, it happens occasionally. It's been happening a lot in our recent campaign - we're playing Return to the Temple of Elemental Evil, and the foes in that can be pretty static, especially the goons. So we go from fights with frighteningly tough high level clerics, demons, or dragons, and in the next room are 6 ogres in full plate. Not advanced, leveled, half-fiendish ogres. Just ogres. Those fights are sometimes fun, but after a while they get dull. There were dozens of 1st level human warriors in another section of the mines, with pretty good gear. After a while we started refering to them as "money makers" because they carry gear that we can sell for a lot more than the effort of swatting a 7hp goon really warrants.