Based on this Apple support page, the iMacs (in particular the 2014 5K I just ordered) do not support parity checking on the memory. From other articles, it appears ECC is not supported either.
I find this surprising. If there is a memory error, then the computer just goes along its merry way, propagating the error until it happens to cause some other detectable exception (like accessing an invalid memory address or running an invalid instruction). Though it might not, and simply result in incorrect or corrupted data.
I often do large numerical simulations, for which such an error would probably not cause a crash, but could likely propagate to a large error in the result.
You don't even need a cosmic ray to get an error. The drive to pack to higher and higher densities inevitably results in vulnerabilities, such as row hammering. One study measured an average rate of four errors per year per DIMM.
What is the rationale for no parity checking? I can see that ECC might slow things down a little and cost more, but at least a detected parity error could bring the system down instead of propagating an error silently. 65 bits instead of 64 bits would be a very small price to pay. Even if it would make sense for a consumer to not have parity checking, I would at least like the ability to replace the memory to get parity checking. It seems that even the DDR3 standard doesn't support just parity checking -- only ECC.