Microsoft Excel Patching
The (original) Microsoft Excel Equation Editor was written in 2000 by a vendor team called Design Science, Inc.
That source code was lost (at least to Microsoft), leaving behind only the final source code.
In 2017, a vulnerability was found in the original equation editor.
Without the code, Microsoft resorted to manually patching the binary executable. They reverse engineered the code and made the patch in pace using the exact same space occupied by the original instructions.
The whole writeup is pretty impressive.