The interpreter (scala) allows shebangs for convenience. For example:

Hello.scala:

#!/usr/bin/env sh
exec scala "$0" "$@"
!#

object Hello {
def main(args: Array[String]) {
println("Hello World!")
}
}

$ chmod a+x Hello.scala
$ ./Hello.scala
Hello World!

However, the compiler (scalac) strangely does not accept shebangs.

$ scalac Hello.scala
Hello.scala:1: error: expected class or object definition
#!/usr/bin/env sh
^
Hello.scala:2: error: expected class or object definition
exec scala "$0" "$@"
^
Hello.scala:3: error: expected class or object definition
!#
^
three errors found

This creates an artificial distinction between Scala scripts and Scala programs, forcing programmers to choose one or the other. And when a programmer uses someone else's code, he'd rather not have to manually add or remove shebang lines to make scala and scalac happy.

Fortunately, the solution is simple: Have scalac consider shebang lines as comments. Anything between #! and !# is just a comment.