How well do you know your bike. If you spend hours and hours on it, it should be like your best friend, your life partner, your third arm. In other words, you should know it pretty damn well. Close your eyes, do you see the spot on the head tube where the paint is worn through by the cable housing? Do you see that chip in the paint on top of the bb shell where the 8mm wrench suddenly broke the pedal free and smacked the shell? Do you see that your derailleur hanger is straight and it, along with the cage of the derailleur is perpendicular to the hub axle?
Yeah, didn't think that was one that is one you're too familiar with. What I'm getting at is if you know your bike, you'll know when something is amiss with a quick look/inspection of your bike. Heck, anytime something happens to your bike that is not in the normal course of a ride, you should inspect it. Did your bike fall over when you parked it to go in the bakery and buy a scone and coffee? It did? Then inspect it. Look at the side it fell on to. Look at the brake/shift lever. Is it askew? Look at your bar end shifter. Does it still rotate in line with the drop portion of the bar? Look at your pedal. Is that only a scuff on the side of is something missing? Look at your derailleur. Is it hanging off the derailleur hanger completely vertical with the pivot bolt and cage perpendicular to the hub axle? No? Whew, glad you caught that one. That could have been a costly repair if you hopped on your bike and started riding and finally got to the largest cog only to find your derailleur cage shifting into the spokes and slowly being pulled back around the cassette to a point the replaceable derailleur hanger snaps (that's what's supposed to happen, right?). But your hanger didn't snap until the derailleur cage was mangled by the spokes/cassette and your chain was twisted into a pretzel.
Yeah, that happened over the weekend. Bunch of guys on a sagged tour to the Mexican border. One guys bike falls over at the bakery so to check if all is okay, he hops on and starts pedaling, shifting through the gears until...kapow, what the... Next thing you know there's a mangled pile of derailleur and chain dangling from the bike. Bike comes to me. "Can you fix this?" Not without the replaceable derailleur hanger. I've got an emergency hanger that will get you home, but in your case, home is 600 miles away. Luckily, Fairfax Cyclery has the hanger, but not the Ultegra derailleur. So, I set him up with the Ultegra derailleur and chain and off he goes to Fairfax for the hanger. Could have been worse, though. Could have very easily gotten the derailleur jammed into the carbon seat stay and snapped it. That would have ended his ride, but he was lucky.
However, luck shouldn't have had anything to do with this situation. After the bike fell, a quick inspection would have revealed a bent derailleur hanger that could have likely been straightened. And even before the bike fell, it probably should have been parked in such a manner that it couldn't have fallen.
One final note: If you have a bike that has a replaceable hanger, it's a good idea to keep a spare with your spare tube. You just never know when you'll need it. It's not a case of "if," it's "when."
If your hanger and derailleur look like this, get it straightened before riding...
...or you'll end up with this - or worse.
(What's playing: Tom Petty It's Good To Be King)