After getting older, but not necessarily wiser, I've since learned what works best for me is to tell the user to "show me what it is
doing". This way, you're not admitting anything, nor are you denying anything.
A very sage piece of advice
I'm into my second decade of programming and engineering. And a few things still stand true:
the basic defect rate is still around one error per hundred lines of code.
Industrial strength testing and review will get about 99% of those, which means each year of coding I leave about one real bug. Somewhere.
Engineers are socially inept.
Conjugate buggy: my design is perfect; your design is interesting; his design is broken; their design is crap.