Of course things can fall thru. The cost of going to court is massive, trying to force someone with no money (mortgage) or assets to fulfil a contract would be foolhardy. You just keep the deposit and try and sell again ....
If there's an unconditional contract of sale in place, the buyer can compel the seller to complete the sale — i.e. to execute a transfer of the legal title to the property to the seller. For that matter, the buyer can compel the seller to execute a transfer of the legal title to a third party, nominated by the buyer.
In other words, although the seller is still registered as legal owner of the property, he doesn't have the ordinary rights of an owner. He's not free to do what he wants with it; he has to do what the buyer tells him. In some respects, he's in the same position as the executor of an estate - has the legal title to the property, but is legally obliged to transfer it to someone else, or as someone else directs. In situations like this lawyers say that the seller is the legal owner of the property, but the buyer is the beneficial owner.