You might observe functions that seem to enter but never return or vice versa. Such rare functions are generally hand-coded assembly routines that branch to the middle of other hand-coded assembly functions. These functions should not impede analysis: the branched-to function must still return to the caller of the branched-from function. That is, if you enable all FBT probes, you should see the entry to one function and the return from another function at the same stack depth.