Websites invariably fail to accept my postal code because of one of two issues: I did put in a space, or I did not put in a space.
It's mind-blowing to me that website developers can't come up with a solution to this problem other than (mistakenly) telling me what my postal code should or should not look like, when the rules are pretty simple.
The other cardinal sin: Canada Post will sell you a gigantic database of postal codes and associated addresses, so you can look up an address for a given postal code. In many cases a postal code will tell you a specific street, specific side of the street, or in the case of condos or apartment buildings, will identify a specific building, so this is great for making sure the delivery address makes sense.
Unfortunately, new buildings and their postal codes are being created all the time, so I've had a few cases where I couldn't order something online because my (three year old) postal code wasn't in their database, so they just gave me an error saying "check your postal code and try again". I ended up having to get a friend of mine to order my wife's birthday present once because it wouldn't accept my postal code for either billing or shipping.
All of this because someone thought they'd be smart and fucked it up.