I've had that a few times on my Smok tanks - VCTs and GCT - as when I first started out vaping those were the only tanks I had, but I wanted to do building, so I was rebuilding the coil heads.
I got too fancy and was trying to put big twisted wire coils in there and all sorts, and had a few shorts.
My iStick 50W protected me a good deal, but it was never able to cut the power off quite quick enough to stop that burnt smell - when I later looked at the rubber insulator grommit, there was often a visible burn/scorch mark, but it never burnt all the way through.
Incidentally, talking about rebuilding coil heads for nickel, the Smok coils are well worth considering - they have 4 large holes, 3mm each, two for juice and two for air. So they provide good wicking and good airflow, and there's a decent amount of space in there to get a larger coil in (my above experiences not withstanding).
The Smok VCT is a fairly basic tank, but priced accordingly - £15 in the UK, I think $18 in the US - pretty much half the price of the better tanks. The Smok GCT is £20 / $25, and comes with 2 Ni200 coils, and is a bit better constructed.
And they've just released the Smok VCT Pro which I've yet to look at, but I'll almost certainly get it just to complete my set!
The nice thing about the Smoks is they have one of the largest (perhaps the largest) range of different coil heads. Single coil: 1.2ohm x 2 (one with juice control), 0.5 ohm x 2 (one with juice control); Dual coil: 0.4 ohm Kanthal; Nickel: 0.2ohm Ni200. Edit: and then new Smok VCT Pro comes with yet another two coils! - 0.2 and 0.6, with "rectangular vertical juice flow" like the Arctic Horizon I think it is. But no word yet confirming it's compatible with the other coils; surely it should be though.
And then also an (optional extra) RBA deck designed for dual coil but also usable in single coil, with massive juice flow and a juice control ring, making it way better for wicking than the Sub Tank mini RBA. It also has an interesting design, whereby the coil(s) do not attach to posts on the deck, but rather the wires feed down through holes in the deck, and then connect to the main body of the head, exactly like in a stock coil - separated by a rubber insulator grommit. That makes building it rather nice because you have a whole deck to work with, no posts in the way (hence fitting dual coil into that small space).
But maybe that would be sub-optimal for nickel builds, for the "lead length" reasons people mentioned earlier.
Anyway, I do recommend checking out Smok tanks if you haven't already, at least to rule them out as suitable for you. Especially if you're on a budget - getting two VCTs for nearly the price of one Atlantis can be rather useful.
