What steps will reproduce the problem?
1. Define a custom function that uses data passed in from the parameters, as well as data from some other source (such as global configuration data living on a different sheet).
2. Use that function somewhere.
3. Change the data from the other source.

What is the expected output? What do you see instead?
I expect to see the custom function recalculated. If I change the contents of a cell referenced by the function, it is recalculated and displays the correct results. If I change the cell *back* to its original value, the *old* calculation is used, and is not recalculated.

On which browser & OS? Firefox 7.0.1 on Linux (Ubuntu 11.10).

Please provide any additional information below.
You can find an example spreadsheet here: http://goo.gl/fzh3q. The second data source is the Pay Basis sheet. The function in question is called in the last row of the Class Schedule sheet. Fiddle with the second data source and the cells matching the regexp /^[0-9]+-[0-9]+/ to explore the bug.

There are two issues here. Ideally, there should be a way to force a recalculation, since it's not possible to automatically determine programmatically whether a recalculation is necessary. However, if that's deemed unworkable, then at the very least, a change in a cell referred to by a function should unconditionally be recalculated, even if there's already a cached return value available.