case class Wrapper(list: List[String]) {
  def check(pf: PartialFunction[Wrapper,Any]) {
    if (pf.isDefinedAt(this)) pf(this)
  }
}

object Unwrap { 
  def unapplySeq(w: Wrapper) = Some(w.list) 
}

// works correctly
Wrapper(List("a", "b")) check { 
  case Unwrap("a", "b") => 
}

// works correctly
Wrapper(List("a", "b")) check { 
  case Unwrap("c", "d") => assert(false, "c, d")
}

// works correctly
Wrapper(List("a", "b")) check { 
  case Unwrap("e") => assert(false, "e")
}

// works correctly
Wrapper(List("a", "b")) check { 
  case Unwrap("e") => assert(false, "e")
  case Unwrap("c", "d") => assert(false, "c, d")
}

// fails
Wrapper(List("a", "b")) check { 
  case Unwrap("c", "d") => assert(false, "c, d")
  case Unwrap("e") => assert(false, "e")
}

// fails, its not just PartialFunctions
Wrapper(List("a", "b")) match { 
  case Unwrap("c", "d") => assert(false, "c, d")
  case Unwrap("e") => assert(false, "e")
}

Welcome to Scala version 2.8.0.RC2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_20).
Type in expressions to have them evaluated.
Type :help for more information.

scala> case class Wrapper(list: List[String]) {
     |   def check(pf: PartialFunction[Wrapper,Any]) {
     |     if (pf.isDefinedAt(this)) pf(this)
     |   }
     | }
defined class Wrapper

scala> 

scala> object Unwrap { 
     |   def unapplySeq(w: Wrapper) = Some(w.list) 
     | }
defined module Unwrap

scala> 

scala> // works correctly

scala> Wrapper(List("a", "b")) check { 
     |   case Unwrap("a", "b") => 
     | }

scala> 

scala> // works correctly

scala> Wrapper(List("a", "b")) check { 
     |   case Unwrap("c", "d") => assert(false, "c, d")
     | }

scala> 

scala> // works correctly

scala> Wrapper(List("a", "b")) check { 
     |   case Unwrap("e") => assert(false, "e")
     | }

scala> 

scala> // works correctly

scala> Wrapper(List("a", "b")) check { 
     |   case Unwrap("e") => assert(false, "e")
     |   case Unwrap("c", "d") => assert(false, "c, d")
     | }

scala> 

scala> // fails

scala> Wrapper(List("a", "b")) check { 
     |   case Unwrap("c", "d") => assert(false, "c, d")
     |   case Unwrap("e") => assert(false, "e")
     | }
java.lang.AssertionError: assertion failed: e
	at scala.Predef$$.assert(Predef.scala:93)
	at $$anonfun$$1.apply(<console>:11)
	at $$anonfun$$1.apply(<console>:9)
	at Wrapper.check(<console>:7)
	at .<init>(<console>:9)
	at .<clinit>(<console>)
	at RequestResult$$.<init>(<console>:9)
	at RequestResult$$.<clinit>(<console>)
	at RequestResult$$scala_repl_result(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at scala.tools.nsc.Interpreter$$Request$$$$anonfun$$loadAndRun$$1$$$$anonfun$$apply$$18.apply(Interpreter.scala:984)
	at scala.tools.nsc.Interpreter$$Requ...
scala> 

scala> // fails, its not just PartialFunctions

scala> Wrapper(List("a", "b")) match { 
     |   case Unwrap("c", "d") => assert(false, "c, d")
     |   case Unwrap("e") => assert(false, "e")
     | }
java.lang.AssertionError: assertion failed: e
	at scala.Predef$$.assert(Predef.scala:93)
	at .<init>(<console>:11)
	at .<clinit>(<console>)
	at RequestResult$$.<init>(<console>:9)
	at RequestResult$$.<clinit>(<console>)
	at RequestResult$$scala_repl_result(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at scala.tools.nsc.Interpreter$$Request$$$$anonfun$$loadAndRun$$1$$$$anonfun$$apply$$18.apply(Interpreter.scala:984)
	at scala.tools.nsc.Interpreter$$Request$$$$anonfun$$loadAndRun$$1$$$$anonfun$$apply$$18.apply(Interpreter.scala:984)
	at scala.util.control.Exc...
scala>