When you have everything running perfect....  and then you decide to "improve" or "tweak" something and get everything totally messed up.
Not MAME related - but I have been fighting with my Time Pilot for a couple weeks now.  It was working perfectly for months, and then I decided to install the hi-score save kit.  Kit works great, but ever since I messed with the cabinet, I've been constantly fixing something.  The night I installed the hi-score save kit, the sound started cutting out occasionally.  A new sound amp IC fixed that, then a couple days later, I was occasionally losing -5V from the power supply.  I replaced some burnt pins and resoldered the header on the power supply board, next the monitor started acting up.  Right now the heaters aren't glowing, so no picture.  I think there may be some bad solder joints or something on the neckboard (K4600), but I haven't had a chance to look into it yet.  *sigh*