Kevin- you just have to keep an eye out for cards that are not legitimate errors from the factory. they are often "psychedelic" and swirly, and have a kaleidoscope of colors. i don't want to call out any specific ones from this thread, but one or two caught my eye that seem like they were either faded, or touched with chemicals (be it glue, or something else unintentional).
Here's a link i found on the subject:
http://t206museum.com/page/periodical_43.html