I just spent a couple hours searching for a bug in my flow, and I now realized that it was due to one out argument being upper case when being lower case in the outer flow. The value of the out-argument did not get out, but on runtime I did not get the usual error of mismatching arguments.
Example:
Flow A calls Flow B to get a boolean value. Flow B as an out argument “ReturnValue”. When Flow A calls Flow B its outargument is named “returnValue” (note the lower case R). The error is that the value of Argument is not passed out to Flow A, but an argument mismatch error has not been raised to notify me of this.