KTM 640 LC4 Mikuni BST40 jet needle
Left is stock needle with under 20K miles (32,187 km) (claimed) and worn by 0.0385" (0.98mm) adjacent to the slide, which works out to 0.0019"/1K miles (0.03mm/1K km) (probably not a 100% linear relationship). The wear is due to slide guide and slide wear causing the slide to be constrained by the needle in the emulsion tube. In addition, this needle was allowed to spin (as evident by the uniform wear all the way around, rather than just on one side), likely due to the white plastic spacer not fitting tightly.
Left unchecked, the needle can wear all the way through. In the best case scenario, the needle will simply fall into the emulsion tube, and the engine will run extremely poorly if at all. In the worst case scenario, the needle will be ingested in the engine, causing extensive engine damage.
It should be noted that by the time a stock needle has broken off, the slide guide, slide and emulsion tube should have been replaced long, long ago. In other words, if these parts are replaced at the interval they should be, there is no danger of the needle breaking off.
Right is a new Factory Pro titanium needle, which is not susceptible to this type of wear, but will significantly accelerate emulsion tube wear.