Quinn Dunki created an Apple II ROM tool project and posted a great write-up on digging into the ROM of his Apple IIc Plus:
You may recall way back when I started my Apple II ROM Tool project, that I had intended to fix some bugs. That was the whole point of this adventure. It took a bit longer than I expected to get here, but we can finally put our kilohertz where our mouth is and fix some brokeass code. But wait, you might be asking, what bugs are in the Apple II ROM? It’s code that shipped in millions of machines, and had hundreds of thousands of pieces of software built around it. What bugs could there be? Well, sometimes a bug is in the ear of the beholder.
Check out the video after the break.