Fairly similar lines of code seem to compile differently here.

da-mystery-of-unit-type.scala

  def render(s: String, mode: NameMode): String =
    if (s.matches("\\d+")) {
      return render(SafeLong(s.toInt), mode);
      // The line below seems to compile.
    } else if (s.charAt(0) == '-') {
      "negative " + render(s.substring(1).toInt, mode)
    } else if (s.length < 2) {
      val c = s.charAt(0)
      if (charIsAlpha(c)) {
        onesForChar(c)
      } else {
        mode.ones(c.toInt)
      }
    } else if (s.length < 3) {
      val firstChar:Char = s.charAt(0)
      val secondChar = s.charAt(1)
      // This line does not compile. `[error] /Users/jimkang/gcw/zillion/src/main/scala/zillion/Util.scala:82:7: type mismatch;
      // [error]  found   : Unit
      // [error]  required: String
      // [error]       if (firstChar == '1') {
      // [error]       ^
      if (firstChar == '1') {
        if (charIsAlpha(s.charAt(1))) {
          teensForChar(s.charAt(1))
        } else {
          mode.teens(s.toInt - 10)
        }
      }
      
      ...

    } else {
      throw new IllegalArgumentException(s"number is >= 10^3003")
    }

I think this was actually a scope issue. This worked.