Here's a recent one. It's from a function that determines the state to transition to in a state machine.
The C code had grown organically for a while and I noticed some duplicated branches, so I converted it to a spreadsheet approximation of a schematic table to be able to see which transitions I could dispatch most easily (like column B and C, then F, because they depend on the fewest values / have the most blank cells). I moved those branches up front in the C code.
It helped me simplify the function in the way that I wanted without any existing tests failing, and (surprise) found a bug that I added a new test case to cover. I don't think I could have done that if I'd only worked with the C code (because I tried and failed 😅).