Whenever I’m looking through my set-up camera, and I try to position it, I sometimes use Alt and right click to pan it by mistake, and the camera suddenly shoots off into the distance, and waaaay out of my scene. What’s actually causing this and why?
I occasionally make this combo also. Yes, it’s some sort of super zoom in / out thing, but I’ve never seen it documented.
A great way to recover from this problem is editor bookmarks. To make a bookmark, just move the camera to where you’re currently working and click CTRL-1. From now, on whenever you want to go back to that point you can just click 1.
This also works for all number keys.
Thanks for the tip. I wasn’t aware of that. That should prove useful. 
I think you’re getting bookmarks and cameras mixed up:
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Bookmarks: At any time during editing a level, you can press CTRL-1 ( or any other number ) and the system will bookmark that point. You don’t need to be looking through a camera of anything. Just press 1 and you go back to that point. Note these bookmarks get saved with the level.
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Cameras: You can ‘pilot’ a camera by right clicking on it and choosing 'Pilot '.To stop piloting a camera press the eject button ( top left in the viewport ). It looks like a CD eject button.
If you ALT-drag anything you make multiple copies of it, and with cameras, that will lead to confusion…
I’ve just tried this method as the issue was annoying me again and it isn’t working. 
I go into the camera’s view, set it manually or press Ctrl + 0/1, make the camera go stupid again by holding Alt and dragging, press 1, or access it manually, and nothing happens. >_<
It works when not looking through a camera, but it’s the camera’s position I need saving in case anything goes wrong.