Replacing the blinking checkerboard cursor with something else?

[jonINTERNETrelayCHAT@napaVALLEYnet.CLARInet (Jon Bettencourt)]

I wrote a couple of programs that POKEd different values into location 50
and printed out some text, to show how text looked with different bit
masks. When the programs ended, I found that the normal blinking
checkerboard cursor wasn't a checkerboard anymore. The first program
replaced it with a blinking uppercase O, and the second program replaced
it with a blinking underscore character.

Does anybody know why it did this and/or if the cursor replacement can be
controlled? The computer I ran the programs on was a ROM 3 Apple IIgs, but
I've seen this happen on an emulated Apple IIc (maybe Apple IIe) too.

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  Figure 1.1: Most Applications Crash - If Not, The Operating
                          System Hangs

   _________                         __      __       __
  /___  ___/ __ _ _  __ _ _  __   __/  \    / /__  __/ /__
     / /__ \/ // / \/ // / \/ /  / / /\ \  / / o \/__  __/
    / / o  /       /       /  \_/ / /  \ \/ /  __/  / /
   /_/\___/_//_//_/_//_//_/ \___ /_/    \__/\___/  /_/
                            /___/
        Figure 1.2: If you squint your eyes a little...

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