Question: I would like to know to know why UART errors occur, and when one should check for such errors. There is a post on here that asks about handling individual errors, such as overrun, parity, etc... I am clear on why data overrun happens, why parity error happens, but I would like to know what is the root cause. My question is more focused on why these errors could occur (physical reasons), and when one should do make error checking a factor for their application.

So far my program seems to work great (with no error checking), but I know that noise can mess things up. How could I simulate conditions that could cause the UART Rx/Tx ports to fail?

Answer: 
There are several potential sources for [noise](http://en.wikipedia.org/wiki/Noise_%28electronics%29) in any circuit. Some of the most common include:

* Poorly regulated power supplies;
* Switching power supplies;
* Insufficient [capacitive decoupling](http://en.wikipedia.org/wiki/Decoupling_capacitor) of the power rails near the MCU;
* [Inductive coupling](http://en.wikipedia.org/wiki/Inductive_coupling) of nearby electromagnetic sources (including 50 or 60Hz from the mains power; even if the circuit is battery powered, it will experience this interference when close enough to a mains source);
* RF sources near the resonant frequency of a trace on the circuit board, or one of its harmonics;
* Routing of high-current traces on the circuit board near signal lines;
* Etc.

In addition (as @jippie mentioned), clock skew is a very common cause of errors in any type of serial communication that uses a predetermined data rate. If you're using an external crystal and interfacing to another system that can reasonably be expected to be accurate, it's less likely to cause problems. Internal oscillators, however, can have tolerances that are several orders of magnitude worse than crystals, and tend to vary more over temperature ranges. 

There are several basic tests that can be performed on a running system to determine the basic noise (and skew) immunity of your interface, including:

* Freezing (cool the circuit to the minimum rating of its components);
* Baking (heat to the maximum rating);
* Exposure to [EMI](http://en.wikipedia.org/wiki/Electromagnetic_interference):
	+ Set the board on top of the power cord of a running space heater;
	+ Key a CB radio in the near vicinity of the board;
	+ Put the board next to your wireless router;
	+ Use long hookup wire (instead of a properly constructed serial cable) for the UART connection.

There are many others--in fact, there are large testing labs dedicated to [EMC](http://en.wikipedia.org/wiki/Electromagnetic_compatibility) qualification.

In general, unless some minimal level of data loss is acceptable, it is always prudent to include some sort of error checking in your communications code. Even a simple checksum is better than nothing.
In order to deal with these errors, you must implement a higher level logical protocol. something akin to TCP, or check the OSI stack for ideas. 

basically, two important parts to start with are checksums, and timeouts. use an algorithm to compute a redundent value that represents, in a smaller form, the contents of each message. then check this in the received message. if the sums don't match, you have possibly gotten a framing error, bit noise, etc, etc. and you'll need to discard the message and attempt some sort of recovery, resend, NACK ( not acknlowledged )signal, etc.

also, make sure to implement timeouts in your upper level protocol. if you get some kind of framing error, your UART may never recover and begin processing again. it may be waiting for the stop bit on a frame that the sender UART thinks has already been sent, but was corrupted by noise , clock skew, etc. this will send any input code into an infinite loop. make sure that you have a sane limit as to how long your input reading should wait until deciding to abandon this message, and again, retry, NACK, abandon, etc.