42S Registers wacky in fast mode

[brh]

Hello all!

I've lurked here on and off for a bit, having used HPs my whole life and maintaining a small but serviceable fleet of calcs. The 41C was one of my earliest introductions to programming, and has always had a special place in my heart, save for its awkward batteries and bulk. So, my go-tos have long been Voyagers and Pioneers. I don't know why I never picked up a 42S until recently; but now I have one, and I love it!

Strange little question, however. It's a ROM A, and I decided to mess around with the so-called 'fast mode.' I have noticed, after doing the instructions for fast mode under program control, my (numbered) registers act all crazy. Regardless of whether I'm in fast mode or not.

If I do 1000; ENTER; STO 01; RCL 01, I get x: -6.62577767124E153
If I do 1000; ENTER; STO 02; RCL 02, I get x: "nAüdAef≤÷"
If I enter something and attempt a SIGMA+, I get an 'Invalid Type' error.

Haven't figured out how to get things back in check aside from a full reset... Anyone have any clues?

Cheers - Brian