I have a couple.

Bally 35 era manuals

Not as much a mistake as a complete oversight of how to make a good manual for troubleshooting. Meaning systemised way of putting forward the information in tables with overview. DIPs, coils w/ wirecolours and driver numbers, bookkeeping/settings etc.

And speaking of DIPs. Oh boy I wish they have had the concept of "recommended settings". As a common foundation of setting up games for competition now a days.

WPC minimum volume constraint

They should have done like Dataeast. Just let people know from the DMD that low volume potentially makes the game less attractive. The constaint have likely had pinball kickout of more locations than it has ever helped any earnings. And for home play - just pain in the ***.

Blame the Euro RoHS directive (for the need to change, not the quality). As the story goes.