Most BBC Micro games loaded like this: game data would spill into memory mapped video and then dealt with after.

The BBC Micro also allows you to change the address of the video framebuffer. A particularly neat trick was to remap it to 0x0000, so you get to see the OS workspace, stacks, program storage etc. You can watch the bits twiddle in realtime as you do things.

There's another neat hack which would cause the machine to run in slow motion. I don't know how it worked; I can't find any references now. Possibly it overloads the system with interrupts. But under its influence, clearing the screen would take several seconds. These two hacks combined beautifully, letting you see all the details of, e.g., Basic's heap management.

The slow motion thing can be found in BEEBUG, Volume 2 No. 6, p31: http://8bs.com/beebugmags.htm

It does overload the system with interrupts, by making the (normally 100Hz) timer interrupt that the OS uses occur a lot more frequently.

I've done it countless times, and not by accident. Sometimes this is the only way to debug things.