Anyone know what caused this?

It looks like an "IR hotspot", although I have never seen one this small or sharply defined. It is a function of the specific lens design - and Bjorn indicates this lens "sometimes" exhibits the problem. The cause, I suppose, is internal reflections at the IR wavelength.

You were at f/22 in the image above, and IIRC that is a well know fault of the 50/1.8 design at very small apertures when shooting IR.

Bjorn Rorslett's evaluation ...

"IR performance: When used for IR photography on some DSLR bodies, the newer AF versions can show an occasional "hot-spot" in the image centre. The MF lens, or at least my sample, isn't troubled with this at all. I've downgraded this lens a little for IR to indicate the potential issue. So don't tell you haven't been warned."

Interesting. I guess I will have to try this again with my 50mm F/1.8 AI.

You've just learned one of the cardinal rules. Never shoot IR at f22.

• f22 has too much diffraction on IR. Heck, f22 is slightly past the diffraction limit for visible light, and you get a lot sharper pictures at f16, for visible light. The typical IR filter makes diffraction a full stop worse, so if f16 is good for visible light, f11 is good for IR. (see note 1).
• For a lens that forms IR hot spots, the smaller the aperture, the brighter and sharper the hot spot. At f16, the spot will be 40% bigger, but only 70% as bright. At f11, it will be twice the size of the one in your picture, but half as bright.

Now, you lucked out. Your spot is so small and so perfectly placed, that it's an easy shop-out. It's hard to tell what doubling the size but halving the brightness would have done.

OK, I mentioned "one of the cardinal rules". That implies that there's more than one...

Another rule: get the best hood you can find. A lot of the "energy" that "powers" the IR hotspot comes from outside the image. The lens hood that Nikon recommends for a 50mm f1.8 is round, and too short, and it lets in a ton of non-image light to feed the hotspot. I think it's also infra-white or infra-purple, instead of absorbing infrared, it reflects it, into the lens as stray light.

I shoot serious IR with a Cokin modular hood. I believe 3 sections is right for the 50mm f1.8. The Cokin is also infra-purple, I lined mine with black flock paper from Edmund Optics.

note 1) The normal, visible light "diffraction limits" are calculated for a wavelength of green light, 560nm. When you have a "weak" IR filter (wratten 89b, Cokin 007, Hoya R72) your peak sensitivity is about 740nm on an unmodified camera, and that means the Airy disc is grows 1.32x larger (740nm/560nm), just about a full stop. So f22 IR looks like f32 visible light, in terms of loss of detail and resolution. Shoot at f11. If you're using an unmodified camera with a really strong IR filter (Wratten 87C, Hoya RM90, B+W 093) then you might try f8.

It looks like an "IR hotspot", although I have never seen one this small or sharply defined.

That's quite normal for cameras that have an IR reflecting "hot mirror" filter like the D700 (D3, D2X, D200, D90). They have double or triple the hot-spot intensity of IR-modified cameras, or older cameras that lack a hot mirror, like D100, D1X, D70, D2H. Nikon introduced the hot mirror in the middle of the second generation: D2X and D200 have it, D2H and D70 don't.

Looks like an alien spaceship visible only by IR to me. Get your guns!

That is one explanation. But, I'm thinking that it is a UFO with it cloaking device enabled and is only visible in IR.
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Seriously, that is some good info about the hot mirror.

I have the same problem with a 50mm f/1.8 AF-D, if that helps. I've had it shooting wide open as well.

I have two version of the 50/1.8 Nikkor: one AFD, which definitively gives a bright hot spot for IR, and an early AIS which does not.

I've seen a tendency for a (weak) hot spot with the AFD 50/1.8 even in visible light, if you shoot without a hood on it. So I'll second Joseph's recommendations. Use the longest possible hood at all times.

Sometimes, when a hot spot is less obvious, one shots the lens at its minimum aperture to look for a putative spot since it then will show its brightest appearance.