Pattern Matching of Sealed Classes

Sealed Classes
If the selector of a pattern match is an instance of a sealed class, the compilation of pattern matching can emit warnings which diagnose that a given set of patterns is not exhaustive, i.e. that there is a possibility of a MatchError being raised at run-time.
When applied to the selector of a match expression, the @unchecked annotation suppresses any warnings about non-exhaustive pattern matches which would otherwise be emitted. For instance, no warnings would be produced for the method definition below.

def f(x: Option[Int]) = (x: @unchecked) match {
  case Some(y) => y
}

Without the @unchecked annotation, a Scala compiler could infer that the pattern match is non-exhaustive, and could produce awarning because Option is a sealed class.
Read more: A Tour of Scala: Sealed Classes