Continuity mistake: We find out in this episode that Ted and the mother don't get married for several years, due to postponing The Wedding when she became pregnant. They eventually get married in 2020, when their children are 3 and 5. But in Lobster Crawl (8-9), at the end of the episode Lily and Marshal show up to babysit Ted's daughter when she's younger than that, and Ted is wearing a wedding ring. (00:29:00)
Suggested correction: Many couples wear rings before their actual wedding as a display of commitment. A friend of mine is in this situation. He wears a ring, has a child with his lady, but no actual date for their wedding is set. This is simply Ted choosing to wear a ring before getting married and there is no mistake.