I think the way WP34s switches from FIX mode to SCI or ENG on overflow and underflow is not quite right.
For example, FIX 04 shows exactly 4 digits after decimal point. As the number grows bigger, it keeps showing 4 digits after decimal, but all digits before decimal. As number keeps growing and there is no more room for digits after decimal, they are getting dropped: first 3 digits are shown, then 2, then 1. All this time it shows all digits before decimal -- so far so good. Then suddenly it switches to sci or eng and shows only 5 digits. It think this is wrong, at least no other calculator does it. At overflow it should switch to unrestricted eng or sci -- just the way ALL mode does. The numeric parameter of FIX should apply to the FIX-specific behavior, not to fallback behaviour.
Another glitch is what happens at underflow: in, say, FIX 02 numbers less that 1e_3 are shown in SCI 02 or ENG 02. I
think this is incorrect. It should show 0.00 or -0.00. Or just 0.00, if -0.00 is too much trouble. When people select FIX, it is to not see smaller fractions of 10, whether because they do not care, or because it does not make sense, like dollars and cents.
At least, this is traditional interpretation of FIX mode by other calculators.