I believe I've found a bug in the FrequencyTimer2 library linked from the Code Library in the playground on the Arduino main page.
The function being called by the SetOnOverflow method is being called at exactly twice the rate as I'm setting.
If I set a period of 10 milliseconds, the function is called every 5 milliseconds.
I'm wondering if this arose from the default functionality of automatically toggling a pin at twice the set frequency (one toggle high, one toggle low), so the coder set his timer to trip twice as often as it should which messed up the function call.
The playground documentation is unclear but it looks like the code is designed to toggle so it does divide the period by 2. If you don't want this, you could just double the period in your sketch or comment out the code in following line in setPeriod:
period /= 2; // we work with half-cycles before the toggle
I don't know if the author (jims) is still hanging out here but if not you may want to add a sentence in the playground clarifying that if you don't expect the handler to toggle then the period needs to doubled or the source code modified as above.
the documentation says:
setPeriod(unsigned long microseconds)
Set the period to the specified number of microseconds. The lower bound is 1, the upper bound is a function of your clock rate, 2^18 clocks. It works out to about 30Hz on a 16MHz clock. If you think in Hertz, then pass in 1000000L/hertz.
setOnOverflow( void (*func)() )
Set the overflow function. This will be called at the end of each cycle. Set it to 0 to disable the interrupt.
So there is nothing in the spec about the frequency doubling. Adding something to the wiki to clarify will just, imo, explain the bug so future users won't have the trouble I did.