I think one can also argue that if you call --translate --validate and have plan files lying around, translation and validation is what you should get. It's hard to see which other behaviour the user expected if they intentionally chose these options.

But it also makes sense to behave as you describe.

I believe the following unexpected behavior is related:

When calling Fast Downward with the driver arguments "--translate" and "--
validate" (without "--search"), the current plan file (default: sas_plan) is 
validated, even though no plan is actually created -- sometimes such a plan file 
does not exist, or more critical, it may be an old one lying around. We probably 
only want to call validate when "--search" is part of the driver arguments and we 
have PDDL files.

Thanks for the report! I'll call this a bug. :-)

When passing the --validate option to fast downward and instead of PDDL files, giving an output.sas file as input, the driver is calling validate with the following options:
validate auto-detected-domain.pddl output.sas sas_plan

Obviously validate complains when trying to read the output.sas file.

It would be good if the driver instead complained that the --validate option and an output.sas file as input are incompatible. 
The easiest way to achieve this would probably be to use the _looks_like_search_input() function from arguments.py to check that the input file is not PDDL before calling validate at the bottom of run_components.py.