Camelina sativa is a weed, and one that has always liked to live in flax fields. Frustratingly for flax farmers, C. sativa looks a whole lot like the real crop, to the point that it's called "false flax." How did this happen? People made it happen.
Weeding out false flax artificially selected for the plants that looked most like real flax, which therefore were missed. And now we have a weed that is increasingly difficult to identify as such. This is an example of Vavilovian mimicry, after the Russian botanist Nikolai Vavilov.