I discovered what I think is a bug in my HP 42S.

Suppose you want to figure the number of Combinations of 8 things taken 5 at a time. But you mistakenly enter the arguments in opposite order:

5, ENTER, 8, COMB

You get an "Invalid Data" error message. Fine. So you press X EXCHANGE Y to put them in the right order. The display confirms 8 in the Y register and 5 in the X register. But if you press COMB you still get the "Invalid Data" error!

In fact the calculator seems to get stuck in a mode where ANY inputs to COMB give "Invalid Data" error. Clearing X, the whole stack, power cycling don't get it out of this mode. The only way I've found to get back to normal is to perform some arithmetic.

For example, multiply the 5 in the X register by 1:

1, MULTIPLY, COMB

then gives the correct answer of 56.

Anyone else seen this?