My Holy Grail used to be something I always coveted but had never even seen before. It was a Milton Bradley eXpansion System for the TI-99/4A. There were several games made to be played with the MBX, but I never saw the actual system outside of catalog photos. It was a voice recognition/synthesis module with an external keypad and a space-age analog joystick. I've read some statements from people who think as few as 300 MBX units were ever made, since it was put out right around the time TI discontinued the 99/4A.
One day I was checking out the local flea market and the collector guy's booth at the back of the place, and I did a double-take when I saw the MBX, boxed and complete. I took it to the seller's register, and he smiled and said "I was wondering how long it would take for someone to realize it was on the shelf". He charged me $40 for it, then said "you realize you're getting a fantastic bargain?" Oh yes, I knew it.
The thing is, I still have yet to try it out. I wanted one since 1983, and now that I have one, I don't want to play it and be disappointed that I wasted so many years wanting it. It's like when I bought a boxed Bally Astrocade and read up on it, with it's high failure rate after all these years, so I put it on a shelf and never played it because I wanted to believe I paid $40 for a working console and I didn't want to know if it was dead on arrival. Eventually, I broke down and played it (it worked). I'll do the same one day with the MBX.