I've seen some questions very similar to this one (http://stackoverflow.com/questions/15483309/org-slf4j-helpers-noploggerfactory-cannot-be-cast-to-ch-qos-logback-classic-logg)), but none of them received a good answer, or at least one that explained or solved this problem

I was able to create a very small project (basically 2 Scala classes - each with a logger - and 2 test classes) with a similar structure to my real project where I found the problem. The example is available here: https://bitbucket.org/pedrorijo91/logger-exp

The problem happens when I run `sbt test`, resulting in the following error:

    {14:43:41} (#47) ~/Desktop/logger-exp/log-exp@pedrorijo(master) $ sbt test

    Java HotSpot(TM) 64-Bit Server VM warning: ignoring option MaxPermSize=384m; support was removed in 8.0

    [info] Loading global plugins from /Users/pedrorijo/.sbt/0.13/plugins

    [info] Loading project definition from /Users/pedrorijo/Desktop/git/scala/logger-exp/log-exp/project

    [info] Set current project to log-exp (in build file:/Users/pedrorijo/Desktop/git/scala/logger-exp/log-exp/)

    [info] HelloTest:

    [info] Exception encountered when attempting to run a suite with class name: HelloTest *** ABORTED ***

    [info]   java.lang.ExceptionInInitializerError:

    [info]   at HelloTest$$anonfun$1.apply$mcV$sp(HelloTest.scala:8)

    [info]   at HelloTest$$anonfun$1.apply(HelloTest.scala:8)

    [info]   at HelloTest$$anonfun$1.apply(HelloTest.scala:8)

    [info]   at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)

    [info]   at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)

    [info]   at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)

    [info]   at org.scalatest.Transformer.apply(Transformer.scala:22)

    [info]   at org.scalatest.Transformer.apply(Transformer.scala:20)

    [info]   at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166)

    [info]   at org.scalatest.Suite$class.withFixture(Suite.scala:1122)

    [info]   ...

    [info]   Cause: java.lang.ClassCastException: org.slf4j.helpers.SubstituteLogger cannot be cast to ch.qos.logback.classic.Logger

    [info]   at com.example.Hello$.<init>(Hello.scala:8)

    [info]   at com.example.Hello$.<clinit>(Hello.scala)

    [info]   at HelloTest$$anonfun$1.apply$mcV$sp(HelloTest.scala:8)

    [info]   at HelloTest$$anonfun$1.apply(HelloTest.scala:8)

    [info]   at HelloTest$$anonfun$1.apply(HelloTest.scala:8)

    [info]   at org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)

    [info]   at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)

    [info]   at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)

    [info]   at org.scalatest.Transformer.apply(Transformer.scala:22)

    [info]   at org.scalatest.Transformer.apply(Transformer.scala:20)

    [info]   ...

    SLF4J: The following set of substitute loggers may have been accessed

    SLF4J: during the initialization phase. Logging calls during this

    SLF4J: phase were not honored. However, subsequent logging calls to these

    SLF4J: loggers will work as normally expected.

    SLF4J: See also http://www.slf4j.org/codes.html#substituteLogger

    SLF4J: com.example.Hello$

    14:43:52.846 [pool-6-thread-4-ScalaTest-running-WorldTest] INFO  com.example.World$ - LOGGING

    [info] WorldTest:

    [info] - test

    [info] Run completed in 456 milliseconds.

    [info] Total number of tests run: 1

    [info] Suites: completed 1, aborted 1

    [info] Tests: succeeded 1, failed 0, canceled 0, ignored 0, pending 0

    [info] *** 1 SUITE ABORTED ***

    [error] Error during tests:

    [error]  HelloTest

    [error] (test:test) sbt.TestsFailedException: Tests unsuccessful

    [error] Total time: 1 s, completed Aug 2, 2015 2:43:52 PM

but if I run each of the tests individually through:

`sbt "testOnly HelloTest"` and `sbt "testOnly WorlTest"` I get no error.

As far as I've seen this is not just a Scala issue, it seems that in Java is also possible to happen. Furthermore, I've read that during initialization the loggers are substituted by a  Substitute Logger (http://www.slf4j.org/codes.html#substituteLogger).

But I can't understand:

 1. why it only happens in tests, and when running both tests? 

 2. why it happens?

 3. how to debug/solve this problem?

 As I said before, I've created a repository with a working example (working, meaning with the error happening) here: https://bitbucket.org/pedrorijo91/logger-exp