Almost all problems in QUANTUM ESPRESSO arise from incorrect input data and result in error stops. Error messages should be self-explanatory, but unfortunately this is not always true. If the code issues a warning messages and continues, pay attention to it but do not assume that something is necessarily wrong in your calculation: most warning messages signal harmless problems.
For Linux PC clusters in parallel execution: in at least some versions of MPICH, the current directory is set to the directory where the executable code resides, instead of being set to the directory where the code is executed. This MPICH weirdness may cause unexpected failures in some postprocessing codes that expect a data file in the current directory. Workaround: use symbolic links, or copy the executable to the current directory.