I'm using mouse. I wanted to drag and swap an item, but I took a wrong one, so I just dropped it below my item slots. For some reason the engine I held was swapped in! The exact place where I dropped it is shown in the screenshot and I can repeat it from the attached save, as long as it's Lgt. Angular Momentum Engine replacing Imp. Deuterium Engine.
I have noticed this odd behaviour at other times, but I was just clicking fast, so I wasn't 100% sure it wasn't my error. Sometimes a utility accidently swaps into some other utility's space and it can probably happen with other parts.