Delving into textual criticism can be safe and rewarding -- but stay alert! You sadly don't have to look for examples. How many people have read this note in the ESV:
"Some of the earliest manuscripts do not include 16:9-20."
However, as many have pointed out, the long ending of Mark is supported by some 1,653 Greek manuscripts, whereas the reading that ends the Gospel in 16:8 has the support of but 3 Greek manuscripts, only 2 of which are "early." This is just my opinion, but it seems to me that Aleph and B may have gotten it wrong here. It wouldn't be the only place.
For what it's worth, I would teach and preach from the last twelve verses of Mark without any hesitation whatsoever. Also, in addition to the Great Commission of Matt. 28:19-20, I freely quote, as Scripture, Mark 16:15: "Go into all the world and preach the gospel to every creature." In fact, this is one of my favorite verses in all of the Bible.
Go everywhere and tell everyone!!!!
Have a wonderful day!