Never go out with a camera you've not familiarized yourself with. I did this once with a borrowed 5D Mark II, and 2/3 of the way through the shooting day (thankfully, nothing deadly important, but still...) I've found that the owner has set a special picture style - Canon Video X I think - that killed all saturation and color, and the pictures looked like putty. Picture Styles! Who would've known... I never use them anyway so I didn't check this setting before shooting.
A more harmless example was borrowing - for a few shots thankfully - a camera which was set to shooting RAW... Ended up with a bunch of files I didn't know what to do with at the time.
Checking all the switches on the lenses, and the mode dial (if the camera has any) is a must as well... When shooting in Singapore, I ended up with a 10+ sequence of slightly OOF shots because the AF switch on my Sigma 14/2.8 is so very easy to knock into the MF position. I should've taped it as I did with many other lenses. Of course, there goes the myth (that I never espoused anyway) of "you don't really have to focus a wideangle" since these were landscape shots but they were demonstrably out of focus and utterly useless. Thank God for the fact that I checked them at the spot - and had the opportunity to re-shoot - and not at the hotel.
My wife often used to end up with botched exposure modes on her Canon 650D because the mode dial is not lockable. She shoots in AV but sometimes found herself in M or TV mode with ridiculously exposed or grossly diffraction-blurred shots. I also advised her to tape the dial but instead she took the better way of checking it when shooting.
And batteries... I can write a horror book on power issues that have plagued me over the years (cellphone, camera, laptop and whatnot...) Think "leaving the charger for the backup body at home when going on a trip, hoping that 3 batteries would be enough - it's a backup body after all". In the end I found that I must never skimp on batteries, whatever I do. Thankfully, it's become really hard to mess something up since I got 1D Mark IV because the Canon LP-E4 batteries are ridiculously powerful... 2000 shots on a charge is nothing out of ordinary for them (unless I shoot star trails and whatnot, of course, because a 45 minute exposure is not the same as a 1/1000 second one. ) Even the no-name spare that I have is similarly tough.
And yes - always handle your gear with care. Don't juggle stuff around when putting it on/taking it off, it's far too easy to drop something. This past April, I set myself back nearly $90 by dropping the camera on the concrete and badly denting the vertical grip side. Thankfully, what I've damaged was a separate ($44) and easily replaceable part - the good folks at FredMiranda.com have looked it up on EBay for me and I have both a pal who deals on EBay, and a knowledgeable repair man who installed the part. But if I had not been trying to do what I did - and yes, people, backpacks are a bad idea! - I would've still had these $90.
There's a part that stems from the last one - treat your gear well. The other shooter in our office doesn't - he goes out in the rain shooting without filters, throws the cap-less lens in his backpack, and whatnot. Not only he's putting himself at the risk of finding it broken - he'll also have a hard time trying to sell his beaters afterwards. I wouldn't buy his 17-40L from him, knowing what he's doing with it (I wouldn't buy a 17-40 either way - unless at gunpoint - but that's another matter).
P.S. I did have a "no card" situation once as well (or worse still, a "no battery" one once - on my 30D that I had at the time it's impossible to say if the battery is there or not unless you open the door or try to turn the camera on). Now I'm always carrying a couple of spare MicroSD cards in my special pocket so even if I forget to put one in the camera when going out, I can just use another.