Some biomedical sensors generate small amounts of AC signal. An example would be a device used to measure tunnelling currents in DNA sequencing. To enable research on such detectors, one of our clients required a sophisticated analogue front-end oscilloscope probe. The necessary sensitivity was down in picoamperes with at least 1kHz bandwidth.
After a few months of work, KAFTS came out with a prototype to support such research. What is the most surprising, is that among many challenges, the most difficult was the problem of dealing with 50Hz grid noise.