My first assumption was a bad solder joint in my wiring but I inspected it all under a good light source with a loupe and it looked ok. Then when I measured the other one and saw it's Vcc was lower than spec that made me think maybe there was a problem.
I decided to remove the dead one but had an unfortunate SLIP! and damaged the ribbon cable so that was the end of that. Once it was out though I turned it over and checked the SMT parts on the back, they were all firmly in position.
What I did notice, comparing the dead one against the working one, is the dead one had a bit of that kapton tape stuff over the ribbon cable contacts. Looking closely (with the loupe, under a bright light) it looked like something was 'wrong' so I peeled the kapton tape back and discovered there was some corrosion along one of the traces.
I haven't had the OLED in or near any moisture, but it has been kind of humid around here lately. I don't know how fast/slow this sort of corrosion would happen, but I'm wondering if moisture did somehow condense under the kapton, could that corrosion have caused the internal booster to fail?
For what it's worth, my other OLED (the older one) did not have any kapton over the ribbon cable or solder connection, and when I closely inspected that, there was no sign of corrosion.
Here's a macro photo of the back of the OLED:Link to the full-size image.
You can see the corrosion beneath R1 and C4, as well as on the back of the Kapton just to the right. There appears to be more around R4 and C3. You can also see my big horrible Ooops along the ribbon cable. Memo to me, don't use an x-acto knife as a miniature lever. (That's what they make flat-head screwdrivers for.)
After taking the photo, I peeled the ribbon cable off completely and found the corrosion seemed to go under the ribbon cable, so it wasn't just under the kapton. A few of the ribbon cable's contacts were black rather than shiney-brass coloured.
Anyhow, I've swapped my other OLED out of the older project and into the new project so it's up and running again.
Having inadvertantly delivered the coup-de-gras to the poor thing myself, I would not ask for / don't expect a replacement, but I do want to point out this corrosion incase this is something to be wary of.