We had a couple of interesting plants turn up from scattered birdseed.
One thing you ought to watch out for is hemp! There are active and none active varieties - "psycho" active that is - as the more common name for hemp is Cannabis.
You'll usually get the non-active type in birdseed - the fibres from the stem were commonly used for making rope, however nowadays it is quite a popular alternative "green" fibre for clothing and accessories like shopping bags.
Getting back to the point - if Mr Plod happens to visit your garden, he'll certainly be interested in it, and will be happy to accompany you to the local police station where you will arrested and charged with cultivation of an illegal plant. You can actually buy hemp seeds in pet shops, but cultivating it however innocently is against the law.
I've no doubt any innocent person will be able to prove their innocence and more than likely "get off" with it, but it's worth not getting into the situation in the first place. If you see any, pull it up and dispose of in any way you see fit.
In general, it's always a good idea to label all of your plants.I have grown about 5 varieties of tomatoes this year, I don't know which is which.
Identification of 'unknowns' is always a 'fun' challenge.Edible and poisionous plants all look the same to the untrained eye.Beware!