Scalac does not find implicits when the use site is compiled after the definition site. Attached is a project with two files. If the project is built using sbt clean compile, it is fine. Now go into sbt ~compile and modify the Test.scala file (e.g. by adding a whitespace anywhere). It won't compile anymore:

[error] /Users/hhrutz/Desktop/implicits_bug/src/main/scala/implicitsbug/Test.scala:5: could not find implicit value for evidence parameter of type implicitsbug.Preferences.Type[String]
[error]     p.put("value")
[error]          ^

Note that this is not an sbt issue. The same happens if you successively execute:

$ scalac src/main/scala/implicitsbug/Bug.scala 
$ scalac src/main/scala/implicitsbug/Test.scala 

What's worse, when Bug.scala is packaged and exported as a library, the implicit resolution fails for any project that tries to use this library.

Another odd thing is, once I refer to the implicit explicitly, the resolution works in successive calls:

object PrefsTest {
  def test(p: Preferences) {
    p.put("value")(Preferences.Type.string)
    p.put("ja!")  // now you see me!
  }
}