My 7D is about two years old, 15,000 exposures so far.
I've recently encountered a frustrating problem. I try to never shoot above ISO 800, usually at 200 or 400. When I go out with the camera it will sometimes, without my having touched anything, change the aperture to something other than the 5.6 I prefer, or it will change the ISO to something ridiculous like 6400. Yesterday I got caught this way when I noticed after taking about 50 shots that the thing had gone to ISO 6400 and F7.2. My last photo session was at ISO 200 and F5.6, and I didn't touch the camera after that session or before yesterday's session.
My first thought would be to check your battery connections and lens to body connections. Perhaps change your clock/memory battery if it is more than a couple years old, or a least check the connection there as well.
I have had my 7D for nearly a year, and probably 30k actuations, and never had anything like that happen.
In don't have a 7D but check to see if you have pre-set modes. You may be hitting a button that is linked to the pre-set. Could have set-up without knowing as you made other adjustments.
I did this one time and took forever to figure out what was going on.
Thanks for the replies, I changed the clock battery and left it powered off with neither battery in for over an hour. Hopefully that will fix it, I haven't had a chance to use it since.
Maybe the camera's mode dial was switched to a user preset (C1 etc.) like Jefferies1 said. Some people reported that the mode dial often switches when they don't want it to like when bumping in to someone, I don't know ... never happened to me. All I know is that lots of people complained and wanted to have a second 'safety switch' button, or like the 1D's have the buttons.
That's all I can think of that might suddenly change your settings without noticing it, find it hard to believe it might be due to a faulty battery or something.
For the aperture problem, try setting it to locked when shooting
I do have a 7D and there is a Menu item under the default setting (see page 47 of the owners manual) where the ISO speed is set to Auto. I would check through these first. Auto ISO can be in P/TV/AV/M modes. Check on the top display to see if there in an "A" displayed for ISO.
