A transport scrambler (also called a 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")