I know there are other topics on this, but I wanted to document what I have done to fix this problem.

After doing a few LED upgrades to my machines, I realized that 555 wedge LED bulbs
have not been designed to the exact specification of the classic glass filament bulbs.
Specifically, they are missing a little grove that allows the metal contacts in the socket
to fit it and hold the bulb in place.
555LED01.jpg

This results in LED bulbs that pop out at random times and fly across the playfield.
And this forces me to shut off the machine in mid-game and retrieve the bulb.
I hate to take a chance that the bulb will cause a short or get lodged in a something mechanical.

I found a reference to "Poster Tack" in an old post and I used this with great success on my AC/DC premium.
555LED02.jpg

I upgrade the flashers behind the lightning bolts and immediately saw the bulbs pop out during the first game.
After wrapping the bulb and socket with poster tack, the problem was solved.
However the solution was not visually appealing.
555LED03.jpg

I did the same for the blackbird light on x-men and it seems to be holding fine so far.
I used less poster tack and it doesn't look too bad.
555LED04.jpg

The fix for spotlights is even better because that are pointed away from the player and you can't see the tack at all.
It seems spotlights are notorious for spitting out LED bulbs, but this fix solves that issue.
555LED05.jpg
555LED06.jpg

However, I recently tried the poster tack on my x-men GI lighting and I've had lots of issues.
There are a few bulbs that are right behind some rubber that gets lots of action and even
the poster tack will not hold them in well enough.

This bulb seems to stay put.
555LED07.jpg

But this bulb constantly eases its way out of the socket.
555LED08.jpg

Then I had a Eureka! moment when I was looking at how Stern prepares the spotlight shields.
They use a small piece of electrical shrink wrap to keep the shields attached.
I realized this could also be used to hold bulbs as well.
I tried this for the first time yesterday and expect this to be a "permanent" solution.
555LED10.jpg
555LED11.jpg
555LED12.jpg

The shrink wrap looks sooo much better than the poster tack.
I'm really happy with this solution if it works for the long run.

I had heard about those other solutions - silicone, super glue, ...
But I wanted something that could be removed without leaving a trace, or "gum up" the sockets, or result in securing the bulb for eternity.

It was 1/2 inch shrink tubing