Why does my gear slip when going uphill / standing? I have a road bike that I use to commute ~6 miles to work every day.  Some of the portions of the ride are at a steep incline, and I've noticed that when I get up to stand on some of these hills, my gear slips.  

I'm not sure if "gear slips" is the right term, but basically after I stand up my gear switches without me triggering it.  It's very annoying and sometimes painful when I completely lose control of pedaling for half a second, not to mention the different gear setting.

What could be the cause of this?
In order of least expensive to most expensive, you either have


a misadjusted derailleur,
a very worn out cassette, or
a crack in your frame.


If you're not noticing any problems shifting under normal circumstances, I'd odds are good that your cassette is worn out and your chain no longer meshes with the cogs correctly.