Cursed Little Frog

This plant grew up strong in a pot of pansies, etc. At first I wasn't sure if it was a weed or not, but an app on my mobile identified it as Ranunculatus sceleratus or Cursed Buttercup.

Apparently rana is Latin for 'frog' and ununculus means 'little' - the plants grows in swampy conditions so I must be overwatering. Sceleratus means 'cursed'', cursed because the plant contains lots of a toxin called protoanemonin.