This is a short post that is mainly just to solidify a mental note for myself: always collect your wavefunctions. When quantum espresso does a calculation, the wavefunction files that it generates can only be used in post-processing if you run on the same amount of processors. I.E. If you run four nodes with 16 processors apiece, you must run all of your plotting/post processing tools provided by Quantum Espresson on exactly 64 processors. If you dont, it simply will not run.
Most of the time, this has not been an issue for me as I normally quickly run the post processing stuff (such as pp.x or the density of states (DOS) computations) right after my relaxations are done. However, even when doing this I was running into issues with the code running properly. I can also imagine that if you were to walk away from a calculation for a couple of weeks, it may be difficult to remember how many processors you used.
At any rate, I would add the following line to your &control card to make it so you never have to worry about this: