Both specs and sbt 0.11 by default run tests in parallel.
In general, this is a good idea. It utilizes the resources of modern multicore systems and helps to flush out inadvertent multithreading defects.
It is a really bad idea when the tests were not written to be thread safe!
Two separate and a little obscure configuration changes are required.
In the build.sbt add:
parallelExecution in Test := false
With in the specification, add a sequential entry. For example:
class Test extends Specification {
sequential