The 38-year-old daughter of Natalie Wood and Robert Wagner won't face charges after getting busted last month for cocaine and heroin possession -- because cops made a HUGE mistake.

TMZ broke the story ... police responded to Courtney Wagner's Malibu home after receiving a call that someone fired a gun inside the house.  Police say they found cocaine and heroin inside and arrested Courtney for possession.

But the L.A. County D.A. won't be pursuing the case -- because of a little thing called the 4th Amendment to the United States Constitution.  According to the D.A., "Deputies did not have probable cause, consent, nor a warrant to conduct the search and recover narcotics so as a result the case is being rejected because of illegal search and seizure."

The Constitution ... it's a beautiful thing.