Got a bit behind again, here are the latest reuploads. If you want a mind-boggling example of a coder leaving a back door in a machine to cheat out wins, check the Thunderbird video.
Point of order - the pub wouldn't lose a penny from the Thunderbird issue, they just wouldn't make anything. These machines were mainly sold to middlemen 'entertainment or leisure' companies who would site them by contract at permitted premises and change them every 8-12 weeks or so as they would rotate their stock around their different premises. The business at the site would get half the profits as would the supplier, or whatever had been agreed, the business paid for the licenses. These cabinets would pay for themselves quite quickly over a period of months (costing £1000-1500 each at that time) by which point the supplier had paid for them so it was very profitable, after they became 'stale' they would offload them to low-rent arcades for a few hundred quid apiece or private buyers in some cases.
These 'errors' would be spotted by simple intellingence - the leisure supplier would come around to check bulbs and maintain the units on the premises every week or fortnight, retrieve the cashbox, max-fill the tubes (refloat) and divvy the the pub their share which they would sign for and make up 'short pays' or customer chits for tube jams or operator refills (via key) that the landlord/staff etc. had authorized and witnessed. If profits were suspiciously low for more than one visit, questions would be asked to the premises owner/staff along the lines of "Does the same person, not a regular, visit say weekly, buy a drink then play it and leave?" and I even heard of VCR CCTV tapes being requested if they had a decent clear view of this. Another clue would be the 'clicker' (a manual digit counter which would count £1 coins being put through) inside the unit. So say this had reached 8,000 in a fortnight, then the leisure supplier's 'engineer' would know approximately what amount of rake should be in the overflow cashbox for that period for that machine and it's RTP setting.
Refill keys as Jono mentioned were a good idea for the player with the knowledge as in most cases, to avoid call-outs, the premises were given a couple to ensure the unit wasn't out of action for anything other than a few minutes. The unit logged any refills so the engineer wouldn't necessarily think anything untoward with refills unless he or she cross-referenced with the operator to check all the times and amounts tallied with their accounted refills, if any (as the coin capacity on compensated units should normally be sufficient for covering streaks.) So 'unaccounted' refills were a huge giveaway if spotted which often they weren't as the engineer was usually in a big rush to get the next premises on their daily run. If spotted, this would be reported back to manufacturer along with lack of profits and an instruction to 'deactivate' would be issued. This means engineer turns off and other sites are phoned to uplug their similar units (which would not be making money anyway, so no real loss.)
Depending on the size of the leisure supplier this could be a major annoyance as they would have to provide equivalent replacements for some of the units but not all as say they had 40 Thunderbirds across their customers, it wouldn't be possible. Retrieved units would go back to the manufacturer for the obvious analysis along with any evidence and presumably this would be all-hands-on for them to find the issue rapidly. Then the engineer would have to rush around 'rechipping' the appropriate units once a solution has been found. Time is money so all parties would be losing income while this happened. It's pretty obvious that they spotted the first Blunderbird issue quickly and amongst some high-fiving and mutual backslapping at Barcrest rushed out the second chip thinking it was remedied only to later discover they had 'moved' the very same £15 win issue further along the code. it was, as suggested in the video, ineptitude. The process would be similar for club machines and with the amount of cash they could hold, things were a bit more stringent. For example, these would be sited for years in some cases rather than months and at certain RTP settings things like the cashpot/reserve at max would never drop, so when someone exercised a method to nudge it in it would look very odd indeed. Because of the huge amounts these could make for a premises with high usage like ferries or legions etc. a few hundred down would not be noticed as much. So players who had the gen on those were far more blessed than they would have been with the rapid lifespan of the £4.80 ones.
With the advent of the internet (about the last time I played pub fruit machines) and the information within, the manufacturers would have become aware of issues far quicker and closed the door. As chopley says, the nature of big jackpots and thus lack of repeats and streaks in the 4-5x jackpot range means things are presumably tighter now. How did I learn this? Back in the 1990's a guy started in my department where I worked and his brother owned a leisure company which supplied and sited the cabinets, for whom he worked when in between jobs. It was interesting stuff, spent many an hour chatting about it.
