AFAIK, there is no "error correction" on non-ECC memory controllers. Of course, I haven't looked at the electrical & physical signaling spec of DDR3 memory interface, so perhaps there's some error correct somewhere (though, if it's at the signaling level, you essentially get it "for free" from a CPU/performance perspective).
The biggest down side to adding more memory is increasing the electrical load on the memory controller. Each DIMM you add increases the amount of current the memory controller has to drive, which in turn means longer switching times. Similarly, denser memory chips made on the same fabrication technology (e.g. 28 nm) also increase the load on the memory controller.
However, this slow down is minimal, as the memory controller is designed to a spec - which means that it should be able to drive the number of DIMMS specified at the speeds specified. However, out of spec, the loading may definitely play a roll in how fast you can overclock your memory controller - you'll more likely be able to overclock it most when you have the fewest, least dense DIMMS populating the memory sockets.
The bigger issues is that of mismatched DIMM performance. You can only push the memory controller as fast as the slowest DIMM, and the chance of you getting a "dud" DIMM goes up the more you use.
However, all these performance considerations are moot considering the large performance boost you'll get from being able to hold extra data in memory vs disk. To that end, there really isn't a downside to adding more memory - the only question is if adding more will increase the performance enough to justify it's price. Past 8 GB, for most typical home use scenarios, the money is better spent getting other, faster components before getting more memory.
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