Well, I wouldn't even try to have a source code written in two different languages in the same component. But I've seen this problem in someone else's real code, which was probably caused by a not really well-thought migration of Java code to Scala. Basically, a developer replaced one Map with another, fixed all the places where API was different on source code level and a number of tests. However, interoperability issues like this one may be hard to catch in general (even with good test coverage and regular code reviews) and they show the complexity of the problem, that requires certain level of development skills and discipline to do it right.

You are right, this particular example is more of case against reflection rather than anything else. Having said that, Scala maps actually are tricky to use from java. In general scala does a better job in compatibility when it comes to reuse of java code from scala, but not the other way around