You were talking about debugging multi-threaded stuff - there was a presentation done by Iulian Dragos at Scala Days last year on Rethinking the debugger.
This covers multi-threaded / reactive / actor / futures debugging:
New abstractions for concurrency make writing programs easier by moving away from threads and locks, but debugging such programs becomes harder. The call-stack, an essential tool in understanding why and how control flow reached a certain point in the program, loses meaning when inspected in traditional debuggers. Futures, actors or iteratees make code easier to write and reason about, and in this talk I'll show a simple solution to make them easier to debug. The tool I present integrates well with the Eclipse plugin for Scala, and shows how a "reactive debugger" might look like.