Mr_Bill wrote:
I have under-cabinet fluorescent lights. Two
light fixures in parallel, wired to one single pole switch.
Each light fixture consists of 2 13W tubes, driven
by a "ballast".

Today, 3 out of the 4 tubes went dark. Long story
short, all is well after I replaced only *one* tube.

Anybody know what's going on?

I'd guess they're not properly wired in parallel.

Chris