A transporter scrambler (also called a transport scrambler or pattern scrambler) was a device designed to discourage transporter use in a certain area by interfering with the annular confinement beam, causing the transported object to rematerialize in a random pattern. Transport scramblers could be programmed to recognize particular signals and only interfere with enemy transporter systems.
The Cardassians routinely booby trapped their abandoned facilities, such as Empok Nor, with pattern scramblers rigged to activate if they detect a non-Cardassian transporter signal. Miles O'Brien remarked that these devices could be "messy". (DS9: "Empok Nor")