If you answer the question "what was their title?" with "they mostly did X and Y, they were incredible", I doubt that's going to cost them the job (assuming they'd put something like X and Y on their resume). Or at least, I wouldn't want to work at any place where the Levenshtein distance between the title on my resume and the title quoted by my supervisor was a critical part of the hiring decision.
For example, Yahoo lets technical employees choose their own titles, with a default of "Technical Yahoo". My ex-coworkers who chose alternative titles don't seem to have been handicapped in their subsequent careers.
Well, "critical" might not apply... maybe it's just an early filter by HR. This filtering out of people who lie on their resume will have a few false positives, but even the best filters are going to have a non-negligible false positive rate -- it's enough that they mostly work, as long as you're swamped with mediocre and inappropriate applicants.
With thousands of resumes coming in daily for a position (I'm guessing), some semi-automated winnowing must happen, and any test you devise will have some potentially awesome candidate crying foul (though such an awesome candidate is probably not in your pool of applicants today, or tomorrow).
"Hi X, as we discussed I used you for a reference for a Quality Assurance Developer role from our time together at FooCorp. I mentioned our debugging of the priority inversion problem in our server, writing the kernel module, and working with you to integrate scripting. Thanks!"
If you are called about a former employee and can't answer the question "what was their title" with something more concrete than "meh, whatever", you will sound like a fraudulent reference. You're inadvertently screwing over your employees if you don't pick something reasonable and stick with it (modulo actual role changes or promotions due to growth).