invunerability fields

[Nyrath]

In Sir Arthur C. Clarke's "Superiority", the Exponential Field was sort of invulnerability.

What happens to stuff trapped in the field when it's turned off?

Heh. That would be a spoiler.
Try to find a copy of the story, it is collected in quite a few places. You may be interested to know that the story is required reading at M.I.T.

[Ma Deuce]

Star Trek Armada II

It doesn't make the ship completely invulnerable to attacks though. Just reflects something like 70 percent of the damage.

Both Armada games actually. And while the Corbomite Reflector wasn't actually an invulnerability shield, one of the Borg ships had a special weapon called "shield remodulator" (or something like that) that could render a friendly ship invulnerable for about 30 seconds.

[Teleros]

I don't remember that, besides the visuals in both Red Alerts show the target being covered in a red force field for the duration of the effect.

From the RA1 manual:

The Iron Curtain resonates the molecular structure of a vehicle or building, bestowing the “will of Stalin” upon it, rendering it invulnerable for a short period of time.

The RA2 manual meanwhile just said what the thing did, not how it did it.