Interesting topic but also consider Stern's warning in the manual - any slight changes voids the warranty and puts it back to the owner. You may develop a mod and sell it with all good intention of providing a simple install with a clear and concise manual but if the purchaser misunderstands your manual or puts it into an already modded machine where quite possibly it has some dodgy installs performed in it (maybe by a previous owner) - where does the onus sit.

I sell a mod for a 1993 game with a very clear manual and use Comet's plug in matrix system to connect my lighting to the games GI and feature lighting and noted I put in a warning at the back of my manual and always state before buying that people read the manual prior as you do get some interesting questions / statements. Sometimes it is also good to have these warnings just to protect your reputation as there is always one that did not read the manual and then abuses you publicly.

Personally this is my limit of selling mods - bolt in mods with plug and play lighting from Comet Pinball. I see alarm bells when modders sell powered mods that use alligator clips - with vibration they can drop off and potential arc and cause a fire.

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in US games there is an outlet in the game - in the coin box area (in 90's Bally / WMS) and pretty sure that turns off when the machine is turned off - Europe and Aus machines do not have this.

pinballs are one of those products that are serviced by owners or un/authorised service techs and over time have already have mods added etc and once this happens I believe all liability / risk sits with the machine owner.

eg
With a new car - when new you get it serviced at the approved service outlets and once you go to unauthorized Joe Blow around the corner as he is cheaper the warranty is void.
With a TV - break the seal the warranty is void and then it's at the owners risk

Games of the 90's have been serviced and hacked by god knows who over the last 30 years, then new Sterns have a very limited warranty of I think 60-90 days and even installing LED's voids the warranty so they wipe there hands of it - back onto the customer.

I think it would be different and you be potentially liable if you supplied a mod and it instantly blew up a heap of stern boards - but comes down to:
- your manual,
- capability of the buyer,
- the hacks the machine already has or
- a faulty mod
Therefore 2 items sit with you (point 1 & 4) so you ensure the manual is very detailed and almost foolproof and then you bench test prior and maybe include a step in the manual to get the customer to bench testing before going into the game to illuminate that risk of a faulty mod killing a game. The other 2 sit with a the buyer.

Personally I would find a way to design a mod to turn off when the game is turned off. I would do a real detailed manual as well as so warranty void notes in the manual.

I wasn't sure hence stating - pretty sure but I sure a few people hooked up to it with some mods and the power packs turned off when the game turned off.