My ungrounded speakers have a tendency to pop and make noise when they come out of sleep (sleep? on a speaker? fuck you logitec) and every time these "simple" CAPTCHAs come up, even if I pass without solving their logic puzzles, I hear the speakers activate as the Javascript on the page is figuring out what kind of audio setup I have by playing a silent sound file.
The default Anubis config isn't really a problem for any devices I've tried, but the FFMPEG Anubis setup is quite extreme. I seem to be served the extra-difficult Javascript challenge, as well as a high-difficulty challenge, that takes even powerful computers quite a long time to complete.
Could just be countermeasures to the hug of death every website gets when they get linked on HN, though, but someone would need to set up auto-scaling for that.