Eggs are eggs right? I mean you can't really mess up an egg can you?
Unfortunately I'm learning that indeed you can.
Egg shells are porous and as such if you're not 100% careful in handling them, you can poison yourself or affect the flavor of an egg by subjecting the egg to substances you shouldn't.
Ok.. where am I going with this? Eggs can be contaminated, as many of us have already figured out through trial and error; however, pinning down the source of that contamination can be rather difficult as there are many factors to consider.
Is it diet?
Most eggs that we use everyday are from chickens. Chickens which are fed a diet of corn with added things, such as arsenic (parasite control), and coloring agents (makes the egg yolks pretty). Xanthophylls are a source of coloring agents for yolks, which are now being extracted from corn gluten to add to chicken feed. Each of these things plays a part in the manufacture and growth of eggs, and can leave trace amounts in the eggs.
Or is it processing?
Eggs are washed before you get them (if you want to see pictures on why click here or here). Most commercial washes for eggs include a detergent as well as a sanitizing agent. Unscented detergents are used as scented ones can alter the flavor of the egg. Which means most likely some of that detergent is sticking around.
Also if there is a mishap in the temperatures as the eggs are being washed, the inner part of the egg may get cooler and cause a "vacuum" effect on the shell causing whatever is outside the shell to be sucked inside the shell. Producers of eggs are pretty vigilant about making certain this doesn't happen as this can cause bacteria from the outside of the shell to be sucked inward and cause rotten eggs.
However, since eggs are washed in hot water (so they don't create a vacuum effect during washing) they will cool later causing a mild vacuum effect later. So if the detergent and sanitizer aren't washed off correctly, some of those may end up in your egg. The amount of which is so small that its probably completely harmless unless you're eating an extraordinary amount of eggs. However, if you're highly allergic to the detergent or sanitizer, you may run into issues.
Also according to the USDA , "the natural coating on just-laid eggs that helps prevent bacteria from permeating the shell, is removed by the washing process and is replaced by a light coating of edible mineral oil which restores protection." This process is done commercially as well as by small farmers, so if you buy locally you may want to inquire what (if any) oil is used. The oil can be mineral oil or miscellaneous vegetable oil which in some cases may be corn.
Several corn allergic persons have run into problems with certain eggs but not others. At the moment, we're unsure of exactly where the problem lies. Feed or Processing?
As a side note, washing your eggs at home to remove any "corn" that may be on the outside isn't advised. Your chances of making things worse is a high probability.