Throughout the years, there have been several that have found their way into the hands of collectors. I personally pulled several out of packs in 1985. Pulled a couple in 1987. I have seen several from the 1989 set (which also has a ton of blank fronts out there as well).
They happen. They are obviously not happening in as great of quantity as the properly-printed cards, but they are print flaws. And with that, if you're looking to sell the item, you need to find the right collector. (But you didn't ask about that, so I won't get into resale value and the like). They are common. I don't think as many from the 1970s are surfacing these days as are ones from the 1980s.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).