I am running the FreqMeasureMulti example "Serial_Output" straight out of the box on a Teensy LC, and getting frequency measurements that are exactly half of what they should be (as measured on my scope). Now I know how to multiply by 2 as much as the next guy, but I thought this might point to a more fundamental problem.

The Arduino IDE is configured as Board: "Teensy LC" and CPU Speed: "48 MHz".