Won 61 games the year prior to the last lockout, then missed the playoffs in the shortened season.
What we're seeing with the Celtics right now really isn't that shocking. What happened with the Supersonics was even more extreme.
Another example is the Charlotte Hornets; they were a 4 seed in 97-98, and missed the playoffs in 98-99.
After a disappointing 4–11 start to the season, head coach Dave Cowens was replaced by Paul Silas, who led the team ta a 22–13 record to end the season. Despite this, The Hornets failed to qualify for the playoffs for the first time since the 1995–96 season.
Not saying the Celtics will miss the playoffs, but it won't exactly be an unprecedented thing if they do.