When executing a search warrant (authorized by the judicial branch -- check 'n balance #1), law enforcement may only seize items within the scope of the parameters of the search warrant.
Hence, if a search warrant authorizes the search of one's text messages and emails between July 1st -- July 31st, the police cannot go on a fishing expedition through one's entire digital universe. Nor may a search warrant be overly broad -- what the founders deemed a "general warrant."
While executing a lawful search warrant, law enforcement may seize items in plain view that are obviously criminal -- e.g., an ounce of meth sitting on the coffee table.
Law enforcement may go back to the magistrate and ask for a second search warrant. Hence, if there is a chopped up body laying on the kitchen floor, such "new evidence" will provide additional probable cause to expand the scope of the search.
Any party with "standing" -- those folks with an expectation of privacy in the place(s) being searched, -- can file a lawsuit if law enforcement has overstepped their bounds, or if one is charged criminally, a defendant can file a motion to suppress/exclude any evidence unlawfully seized by the authorities (check 'n balance #2)