I bought a plant from a petshop, planted it in my tank. All seemed normal, but today when i looked there is a small yellowish snail in my tank.
I rang the petshop to find out if snails were kept in the same tank, but there saying that there is no way it came from there because they don't stock snails.
I have no idea where they get there plants from, or even know what kind of snail it is.
Would it cause any problems in my tank?
Am i best giving it to the petshop?

even tho all plants are washed and kept snail free IT NEVER WORKS! i work in a pet shop and they just appear in our plant tanks. they do come in with the plants and all you can do is remove it and do what you like with it! if you only have one then it wont be too bad and it might eat a lil algue but if you have more than one which is more than likely then they will breed really quickly! how big is your tank? depending on size you can get fish that eat snails or assasin snails which eat snails if you have an infestion but fingers crossed it wont get that bad!

there's only 1 that i can see, how i didn't notice it before i don't know, it's the size of a thumb nail and is yollow! I have fish in the tank that eat snails, but i think it might be too big, had a good look today, looked in all the pots and stuff much to the dismay of my crab, but couldn't find any. I rang the petshop today and they are now saying it might of come from there because where they ger there plants from, he has apple snails in with them.
I've wanted an apple snail, or an assasin snail but was always wooried because of my crab. i thought he might fight with it.
But so far the snail has managed to keep out of the crabs way.
Could i be lucky enough to have a stow away apple snail?

The crab is not a great mixer with any fish never mind the snail. Bottom feeders in particular could be a target for it.
Stow away snails are common with purchases from shops. It isn't too difficult to keep their numbers under control, just remove them as you see them. I personally would never recommend buying a fish to control snails, you should really only buy fish because you like them. Many fish are bought to control algae/snails and end up in tanks unsuitable to their needs.

As Lawrence has quite rightly pointed out, crabs and fish don't really mix. I've seen Rainbow crabs grab and consume small rasboras that strayed too close (not in one of my tanks however).

Pest snails can come in on live plants, and usually go unnoticed in aquatic stores until the plants are purchased.

I strongly recommend dipping live plants in a solution of Potassium permanganate (KMnO4) at a concentration of 10mg/L prior to adding them to the tank. This substance is a highly effective means of mollusc eradication. Or, copper (often found in several whitespot treatments) can be used to a smaller effect.

a couple of the fish i have in are the snail eaters.
The crab is no bother, never bother's my fish, spends most of his time in his hut. He's a red clawed crab and is pretty small still, all the fish i have in the tank are bigger than him.
The snail seem's happy enough, and i am happy at getting a free apple snail!