When using Unitils with the Eclipse compiler, an "Unable to find indexes of argument matcher" exception is generated at runtime if the method call is on a different line from the behavior.
 
For example, the following work fine (mockConverter is declared as Mock<Converter> mockConverter):
 
mockConverter.returns(convertedDate).convertTo(utc);
mockConverter.returns(convertedDate).convertTo(
utc);
 
But the following throws an exception:
 
mockConverter.returns(convertedDate)
.convertTo(utc);

This appears to be a variant of UNI-105, where an exception was generated at runtime if the argument matchers don't appear on the same line as the method call.