Way back in the early 1990s, there was a programmer named Michael Geary who worked for Adobe. Geary's project was Adobe Type Manager, a software utility that allowed scalable Postscript fonts to be used in Windows 3.0, in the days before TrueType.

Specifically, Geary wrote the code that hooked into the type rendering mechanism of Windows, allowing ATM to do its thing. This involved patching a system DLL, and presumably was fairly difficult without Windows source code and the primitive tools available around 1991.

Geary discussed the solution he came up with publicly on a CompuServe forum, where it apparently attracted some attention.

On October 31, 1991 (Halloween) Adobe CEO John Warnock received a letter from Microsoft. The letter made baseless accusations that Geary had revealed trade secrets. Furthermore, when Windows 3.1 was released, it contained superfluous code to fool ATM. ATM would scan the GDI DLL for specific code after the entry point to CreateDC. It was changed to look like this:
    jmp somewhere_else

ATM would see the decoy code and patch it instead. There is no other explanation for this except the deliberate intention to prevent interoperability with a competing product: recall that Windows 3.1 introduced Microsoft's own scalable font technology TrueType.

Source: Undocumented Windows, Andrew Schulman, David Maxey, and Matt Pietrek.