Weedling or Seedling? Below are weedlings in my book, which means that they are thugs if left to their own devices..

Creeping buttercup, Ranunculus repens

Arum Lily, Lords and Ladies, white flower followed by (poisonous) red berries, a prolific self seeder.

Arum

Celandine, a pretty yellow flower but better in a woodland setting than in the borders, make sure you get the tiny bulbils or it will be back!