I've had to work with some horrible APIs for proprietary software in my time, and I'm interested in knowing other people's experiences.

I'll start with my worst: Photoshop!

-Terrible documentation

-Multiple conflicting APIs

-Super steep learning curve in some parts

-APIs that never escape beta before deprecation

-Cut-down and locked down environments (JavaScript)

-Impenetrable internal scripting syntax

-Key aspects of the application locked out from scripting

Above is with CS6 - I haven't used CC (customer still on CS6 :| )