The elusive negative zero on the HP 30b

[EdS2]

Back in 2012, Katie Wasserman noted something unusual:

Quote:Try -55555 ^ -55555 on your 30b and tell me what you see.

The correct answer is a very small negative number which on most calculators will be rounded to zero. In this particular case - and it seems the 20b doesn't do it - the result is a displayed negative zero, which even tests as unequal to zero, although arithmetically it behaves as zero.

My questions would be
- what other calculations will return a negative zero?
- how did Katie find this one?
- why does the 30b do this? (and why doesn't the 20b)
- do any other HP calculators do this, or have any similar oddity?

Thanks to Bob for checking the 20b behaviour.