One of the differences between APM 2.0 and APM 2.5 is that on APM 2.5 there is no daughterboard covering the barometric pressure sensor. So what? Well, if you've got your APM 2.5 in an enclosure of some sort, it's no big deal (the official enclosures will be out in a few weeks and will be included with APM 2.5 orders then; people who have received an APM 2.5 already are entitled to a free enclosure with their next order). But if you don't have have your APM 2.5 in an enclosure, the lack of daughterboard protecting the baro sensor from the windstream can wreck havoc with your altitude measurement!
I learned this the hard way this morning, when I mounted my new APM 2.5 on the top of my quad, out in the breeze. When I switched on Altitude Hold mode, it decided to head straight for the skies. A quick switch to manual to land, then an upload of logs to the dev team revealed the problem (thanks to Jason Short and Pat Hickey). The vortex from my props was generating crazy readings with baro sensor. (This hadn't caused a problem on APM 2.0 because the daughterboard served as a wind shield.)
Solution: a bit of cotton padding, taped on top of the sensor, as shown above. Back to air and all was well-rock-solid Alt Hold. Whew.
So if you've got your APM 2.5 mounted out in the open, like I did, tape on a bit of cotton padding or a bit of paper tissue for now, until you get a proper enclosure. And remember: pressure sensors are, well, sensitive to pressure ;-) That includes wind of any sort. Shield your sensors!