At all times it should be remembered that this is not a commercial product, developed and supported by professionals. Instead it is a `University World' product, developed by a very few physicists (mainly the current first author) originally for their own needs, and supplied to other physicists on an `as-is' basis, free of charge. (It is protected by copyright, however, so is not `free software' in the nowadays common meaning. This is not intended to stifle research, but to make people respect some common-sense `intellectual property' rights: code should not be `borrowed' and redistributed in such a form that credit would not go to the people who did the work.)
No guarantees are therefore given for the proper functioning of the program, nor for the validity of physics results. In the end, it is always up to you to decide for yourself whether to trust a given result or not. Usually this requires comparison either with analytical results or with results of other programs, or with both. Even this is not necessarily foolproof: for instance, if an error is made in the calculation of a matrix element for a given process, this error will be propagated both into the analytical results based on the original calculation and into all the event generators which subsequently make use of the published formulae. In the end, there is no substitute for a sound physics judgement.
This does not mean that you are all on your own, with a program nobody feels responsible for. Attempts are made to check processes as carefully as possible, to write programs that do not invite unnecessary errors, and to provide a detailed and accurate documentation. All of this while maintaining the full power and flexibility, of course, since the physics must always take precedence in any conflict of interests. If nevertheless any errors or unclear statements are found, please do communicate them to one of the authors. Every attempt will be made to solve problems as soon as is reasonably possible, given that this support is by a few persons, who mainly have other responsibilities.
However, in order to make debugging at all possible, we request that any sample code you want to submit as evidence be completely self-contained, and peeled off from all irrelevant aspects. Use simple write statements or the PYTHIA histogramming routines to make your point. Chances are that, if the error cannot be reproduced by fifty lines of code, in a main program linked only to PYTHIA, the problem is sitting elsewhere. Numerous errors have been caused by linking to other (flawed) libraries, e.g. collaboration-specific frameworks for running PYTHIA. Then you should put the blame elsewhere.