When police accidentally catch a drug user, can they arrest him?

From the Constitution Daily:

This week the Supreme Court heard argument in Kentucky v. King, a case that involves the question of the warrant requirement under the Fourth Amendment before a search can be conducted of someone’s home or apartment.

To get a warrant, police must convince a judge that there is “probable cause” for its issuance. Like virtually every rule in law, there are exceptions. If the police find themselves in an emergency situation – “exigent circumstances” – they don’t need to get a warrant. For example, if they hear a violent argument or gun shots coming from a home, they don’t have to get a warrant. They can break in. However, there are exceptions to the exceptions. If the police themselves create the exigent circumstances, they can’t break in. Or, if they do, any evidence of criminality will be suppressed at the subsequent criminal proceedings.