Like Nino stated.....hard to explain. Some Bingos are quite challenging (thinking wise) to be able to move the screen (e.g. magic screens) to be able to actually locate all the different ways of winning credits. You could easily miss some of them.
Like pinballs with 'High Scores', I like to think the same with the Bingo, but using the 'Credits'.
Getting a large credit win is very satisfying indeed.
 
Hmmm, yes, fixing them can be very challenging. 
 
Those 2 BNIB Tahitis my brother & I bought never worked from new, they had some major issues with them. Which I believe someone knew about from when they were manufactured, hence why they were stashed away in a warehouse ??. There was 6 of them apparently.
Anyway......I love them.