My favorite is the Westside "American Garratt"
The customers stayed away from the slide valve K-27's in droves, so Westside was stuck with a bunch of them. Seems Dick got the bright idea of taking two slide valve K-27 frames, one boiler and cab, one tender shell and making a Garratt out of them.
All he had to do was make a boiler cradle and a front tank. Of course, he or his designer's didn't know the first thing about Garratts, so the pivot points are all wrong on the frames. Also, how a boiler that could produce just enough steam for one set of cylinders was supposed to supply two wasn't something that came up in the desing process either!
Actually I doubt if there was a design process. I think someone just sat at work bench and stuck the parts together. Someday, I'll make a proper size boiler and relocate the pivot points to the correct location in the frame and then I'll have something that at least "might have been."
I won't get into the beautiful and otherwise well made Westside shays with the soft metal gears and the vertical drive shaft, torque-o-matic gear box that caused the second truck to hope sidways and derail as soon as there was any load behind the engine!