I haven't got a clue!

I enthusiastically sewed my seeds, and then realised that I may have been a bit too enthusiastic!
Hundreds of plants to prick out, and this little seedling has cropped up all throughout my beans, leeks, radishes, carrots, spring onions and chillies.
At first I thought that this was a 'genuine' seedling and started to prick them out, but now I'm confused - I think it's a weed!
If anyone can identify it I'd be grateful :-)