As I have already stated in the "First Impressions" thread, the HP 20b has a strange bug. I did some research and could narrow down the circumstances:
1) Select RPN mode
2) Select radian mode
3) Clear the stack (just to make things easier to check out)
4) Enter 1 +/-
5) Select Math/trig and scroll to COS-1
6) Press input
Now, the rounded value of PI is in the display and the argument -1 is in stacklevel Y. You can check it with swap ")" or roll down "(".
This is the only situation I could spot where the argument is not popped off the stack but pushed to Y. In degree mode, the bug is not present.
Marcus
Edited: 17 Sept 2008, 8:22 a.m.