I just put my remaining GE FLB-17 lamp into service last night to replace a failed electronic bullet CFL. The FLB-17 is an integrated twin-tube, 16 watt lamp with preheat-type magnetic ballast - the lamp & ballast don't separate.

I had played with it a few times for short periods but it's mostly been in a box, stored away for some years now. It has maybe 2 hours of total use on it. I installed it base up yesterday and it lit normally. However, the second time I turned it on that night, it wouldn't start - it just blinked endlessly like a failed lamp. I took it out & tried it base down. Same thing - it just strobed away.

Figuring I had a defective lamp, I decided I'd just see how long it would strobe until it died completely, and maybe would make a video. After some 20-25 minutes of continuous blinking though, the lamp suddenly lit & stayed lit!

I reinstalled it in the fixture I plan to use it in and it has worked fine since (so far) though it blinks a second or two longer than the other ones I have, before starting.

Any idea as to what happened?

Normal "burn-in" settling. During the storage internal stuff degraded in rather unnatural way (for the fluorescent), so it had to settle down.
I observed similar phenomenon on regular F36T8 - they appear heavily mercury starved, but after first decent warm-up it operated normally.

I've had batches of lamps with very temperamental burn-in. Once, a case of Sylvania F40 D50's at work had trouble starting after installation. They would just glow dim randomly on different days in RS fixtures but no end blackening. As time went on they worked normally, and after several years, they are still going strong.

Another, I bought a case of Sylvania made Duro-Test Vita Lites in F15T8 size and every one of them wouldn't fire up on a regular 120v PH choke ballast. The ends would just make orange-purplish glow. However, they would fire up on rapid start or electronic quite easily (higher OCV), but would flicker and swirl like crazy for several hours before stabilization. After that, they worked fine in 120v PH, and the longer they are run, the more normal they become.