Ouch !

[paul.creedy]

I think I've found the guilty party - a 9v 500mA switching power supply.

It was running two Line 6 pedals but when I unplugged one and plugged in a Boss GE-7 to test (I was modding it for a customer) the case was live, not temporarily but constantly.

So I've removed it from my set up and I'll use something else for a while to see if the shocks disappear.

[paul.creedy]

Well, if the power supply was responsible, it wasn't the only culprit.

I unplugged a couple of my recent builds to tinker with the paintwork when my (bare) foot brushed the jack plug on the floor which is the beginning of the chain.

Unlike my hands, which have experienced shocks more than once, my foot was unprepared and as a consequence I staggered and nearly flattened all the stands and pedals 

No harm was done though, and I also found out that the power supply might still be useful in emergencies, so all in all it could have been worse

Back to the drawing board !

[EBK]

I just glanced through the posts in this thread quickly, so I may have missed something. But, have checked to see if your house is wired correctly?  I would suspect a ground that is not properly earthed.

[paul.creedy]

I'm generally happy with the static theory as once a shock has been received, the item that dealt the blow is no longer live.

I've not much work on this week so once I've spent a little time seeing if I can get my Parallel Universe working properly I'll take a meter to various points and unplug a few things to see what happens.

[MrStab]

MOSFETs might be less happy with the static theory!

[anotherjim]

I would check the AC power earthing too.
Unplugged from the wall, your instrument jack ought to have continuity to the amplifier AC plug earth prong.
If that's ok, then you have to wonder if the wall socket earth wire is connecting (they can be lose in the screw terminal but keep contact until corrosion stops it many years after installation).
If you have an RCD safety cut out plug adapter and plug it into the wall socket and if pressing the test button makes it trip, then there must be a working earth connection in that socket.

[merlinb]

I think I've found the guilty party - a 9v 500mA switching power supply.
It was running two Line 6 pedals but when I unplugged one and plugged in a Boss GE-7 to test (I was modding it for a customer) the case was live, not temporarily but constantly.

Switch-mode power supplies often have small caps connected from primary to secondary. This means the secondary floats at about half mains voltage (but high impedance) until you connect something with an earth reference. This can indeed give you a small shock when plugging/unplugging.

[pinkjimiphoton]

big freekin caps in the pedals themselve's power supplies will do it. they may not all be at the same ground potential YOU are and when you touch them, they discharge.

i had some pedals on my live board that would literally still be glowing a week after i played a gig i posted about last year.

check your grounds. i had one bad ground on one of the pedals, a jack got loose JUST enough where it wasn't discharging properly. i dunno, i tightened and cleaned everything, but it went away so i guess i got it.

[Kevin Mitchell]

but it went away so i guess i got it.

NOOOO NOT THE PHANTOM LED 
RIP possessed pedal.

-KM

[pinkjimiphoton]

i offer ZERO explanation of WHY it was like that, kevin lol