Welcome to the forum. It is not an error made by the mint, it is a stamp that someone made after it was released into circulation.
Sometimes when the coin is minted the design can be a little off. In fact, there is an type of error called off-center coins where the design is way off of the blank coin. Here is an example: Source: http://coinquest.com/cgi-bin/cq/coins?main_coin=925
post your 1980 counter-stamps here, http://www.cointalk.com/threads/post-your-counterstamps.239383/#post-1825915 theres already a thread started
your 2011 looks like a MAD, mis-aligned die, seeing as how only one side is affected, notice how the rim is thicker near the top an thinner at bottom, but obverse the rim is fairly even all around.... off center strikes affect both sides evenly..