It's normal for most equipment with switching power supplies to have
some relatively studly capacitors across the AC line, center tap to
chassis. This is supposed to be to keep any RFI generated inside the
box from leaking back out through the power cord, and vice-versa.

Older equipment tended to have much smaller capacitors, on the order of
0.005uF, which could still give you a very mild tingle, but nothing
like what you get from todays capacitors-- I've seen up to 0.1uf to
ground, 0.47uF across the line.