Many people get that error (and several other errors) multiple times a test.
The testing booth is authorized around 20 dB or so (depending on frequency) of ambient noise, per OSHA/AFOSH. You think it's louder than reality by the sudden decrease in ambient volume that your perception ramps up. I try to test people in an environment as quiet as possible, but we can legally test you in what you would consider a pretty noisy area (from a tester's perspective). The time intervals are "randomized" (it cycles through 30 or so preset differences in milliseconds between stimulus). You got lucky or you tested well. Many people that do this think they did better, but know that you need to respond TWICE on the same intensity (volume). I often switched to manual testing because I noticed what they were doing. They still thought they were doing well because they stopped getting error messages, but there aren't computerized errors on manual mode. If they kept doing it, I'd open the door and re-educate them
I've given thousands of tests in my career (as have most non-retrainee NCO's); I'm wise to your tricks.
However the dopey E-2 might not be.
The advice I give testers is push the button if you think you might hear it. If you start to get a few error messages, then be a little more positive you actually hear it before you respond.
Also, if you can tell, push the button between the second and third beeps. You're less likely to get errors. Pushing it after the third beep or on the first beep will likely result in the machine not counting that response, but immediately re-testing you on that intensity.