It’s worth noting that a patch around 2 months later removed the Equation Editor entirely, because more security issues were found and binary patching wasn’t going to be sufficient.

Are are more explanations for the manual patch than losing the source code though: why spend days/weeks setting up a 20-year old dev and build environment when a two-byte patch does the trick?

Oh that reminds me of learning some assembler by writing cracks to shareware programs to patch away test period warnings, license key checks, etc. using a disassembler (w32dasm I believe) on the binary executable. That was a fun learning experience, helped a 14 year old to use software he otherwise couldn't buy (me) and helped understanding quite a few internals of binaries.

Some binaries are obfuscated by overlapping several routines together.

Oh, there's a bunch of ways you can try to make reverse engineering harder, but I don't think these techniques are really that common for most software.

Wow, that's impressive.