Sounds like that ballast has a capacitor or condensor (same thing really) to hold a bit of charge with the power off. I'm not gonna disassemble mine to verify that, but a capacitor would be the usual source of a tingle (or jolt, if it's big enough) with the power disconnected.

We used to leave charged capacitors laying around the parts counter with tags reading "do not touch" as a joke. Some kid would pick one up and get bit. The rest of us would laugh our asses off. Those things can hold a charge for days . . .
rodekyll Reviewed by rodekyll on . ballast problem?? hi all hope someone can help me,,today when i went to plug my light in, i touched the two prongs on the plug and received a tingling sensation in my finger,,the light had been off for twelve hours. Ididn't know if the ballast keeps some power in it when switched off. I'm in the UKjust so you all know.. thanx for looking SI Rating: 5