Breakdown- Have you ever felt forlorn? Lonely and abandoned?

Were you feeling lonely because your ride partners pedaled away, abandoning you on the side of the road with a preventable mechanical?  Oh, they'll probably stick around until you have a rescue arranged, and they probably won't talk bad about you until they're out of earshot, but you'll still have your ride cut short.

DISCLAIMER
If your eyes glaze over at the mention of the words "hex wrench", just stop reading now and take your bike to your favorite local shop and ask them to check it over. Likewise, if you're a seasoned bike tinkerer, I'm unlikely to teach you anything.

Miles matter.
Many cyclists will get their bike checked out in the spring as the season ramps up, only to ignore routine maintenance, service checks, and repairs as they pile on the miles through the summer months. I'm going to reach into my bag of recent experiences to point out a few things that could go wrong in the hope that it may save you that dreaded phone call home--"Honey, can you come get me? Do you know where Garrison Road is?"
Don't ignore noises.
Most of us know the sound of a normally running bike and of a dry chain in need of lube. For noises beyond that, make it a point to hunt down the source of clicks, creeks, groans, and squeaks. If the bike doesn't sound right, it probably isn't. Most noises can be corrected with a hex wrench or some lube. Some can't.
Noise can precede catastrophic failure..
Usually, stopping a noise can be as simple as greasing your seat post or dropouts, but it may signal a disaster in waiting. Last year, after riding for a couple of weeks with a tube of grease in my pocket chasing a creaking noise, I discovered that the seat tube on my titanium frame was seriously cracked. Only last week, a ride partner found that the source of an irritating click was a carbon seat post that was cracked and near failure. Either of those could have caused a crash or at least that roadside phone call.
Noises radiate through frames, often making it difficult to pinpoint the source. If you don't feel comfortable making minor repairs and adjustments on your bike, take it to the friendly folks at your favorite bike shop.

It shifted OK yesterday....
A change in performance means something is wrong. Sudden problems with shifting, usually the rear derailleur, can signal one of a number of problems, but the source can be narrowed down pretty easily.
First, ask yourself if your bike has fallen over or otherwise taken a blow to the drive train. Then, make sure that the rear wheel is firmly and squarely set in the dropout.
Next, make sure that the cable end is firmly tightened under the retention bolt and has not slipped. Once properly tightened, the cable rarely comes loose, but it's worth checking. If it is slipping, you will likely get phantom shifts to smaller cogs as tension comes off of the cable, and the cable near the retention bolt will usually show signs of having been flattened. 
The next most likely cause is a failing shifter cable. The cable usually breaks a strand at a time, usually inside the shifter housing, effectively lengthening the cable and allowing phantom shifts to a smaller cog.

This cable is on its way to total failure. Roll back your shifter hoods to check for failing cables.

You can usually shift back up to the desired cog, but as more strands break, the derailleur will drop again to smaller cogs. This often takes place over a period of time and a couple of rides before the cable fails completely.

 

Nothing like a wad o' wire to enhance shifter performance.

My most recent experience with a frayed cable was a little different in that the wad of broken strands prevented the cable from feeding out of the shifter. The result was that I couldn't use my smallest 3-4 cogs.
A failing cable is easy to overlook, even by experienced mechanics, if you are not specific about the symptoms. The shifting can often be adjusted to perfection on a workstand, only to have the problems return out on the road as more cable strands break under the load of shifting. If you suspect a failing cable, check it yourself or ask you mechanic to take a look. Better yet, if your cables are more than a year old, just replace them. They're inexpensive and should be considered a wear-and-tear item like tires and chains.
Cable life can vary widely and can depend on the cable routing. Failures are likely to happen at sharp turns of the cable. The current DuraAce and Ultegra 11-speed groups require several sharp bends to the cable, inspiring Shimano to use more expensive PTFE coated cables for smooth operation. The components work beautifully but I think that cables will need more frequent replacement than on older systems.

More gradual degradation in shifting performance may indicate drive chain wear. That usually starts with a worn chain. If the worn chain is not replaced, it will cause excessive wear to the cassette and, eventually, to the chainring. 

This chain is within spec. A worn chain would allow the .75 tab to drop between the links. A worn out chain will allow the 1.0 tab. 

A chain with excessive wear usually means it is also time to replace the cassette. Most mechanics check chain wear as a matter of routine. If you don't have a chain check tool, it is easily checked with a 12" ruler. This video link shows both how to use the chain check tool and how to use a ruler for the purpose. The host refers to "freewheel", though modern bikes use a freehub and cassette, but the idea is the same. You can replace a chain now or a chain and cassette, and possibly chainrings, later. It usually makes more economic sense to stay ahead with chain replacement.

Tires and flat kit
As always, you want look over your tires from time-to-time to check for cuts or excessive wear. Also, check the contents of your flat repair kit. It's easy to forget using a CO2 cartridge until you need it again.

This is a rainy day post. If you're reading it on Saturday, it is also a good day to check over your bike or to get it to the shop.