The pattern buffer was a key component of transporter systems. The buffer was used to temporarily store the matter stream following dematerialization, but prior to sending the stream to its target. This was done because of the relative motion of transporter and target. By temporarily storing the matter stream, the Doppler compensators had enough time to adjust the targeting scanners.
A matter stream could not be stored indefinitely in the buffer; after 420 seconds, the stored pattern would degrade and the object was lost. The only known occurrence of a person surviving in a buffer longer than the theoretical maximum was Captain Montgomery Scott on board the USS Jenolan. Following the Jenolan's crash landing on a Dyson sphere, Scott, with the help of Matt Franklin, was able to store his pattern in the buffer for 75 years. This was achieved by disabling the rematerialization subroutine, connecting the phase inducers to the emitter array, bypassing the override, and locking the buffer into a continuous diagnostic cycle. Although Captain Scott's pattern suffered less than 0.003% degradation, and was successfully recovered by Geordi La Forge of the USS Enterprise-D in 2369, Franklin was irretrievable, as one of the inducers had failed, causing a 53% degradation in his pattern. (TNG: "Relics")
In 2152, diamagnetic storms saturated with polaric energy were encountered on a planet visited by the crew of the starship Enterprise. The storms interfered with the operation of the ship's transporter, resulting in Hoshi Sato being trapped in its pattern buffer for 8.3 seconds while Malcolm Reed worked to reintegrate the matter stream. (ENT: "Vanishing Point")
A pattern buffer malfunctioning was the leading cause of transporter accidents. In the 2270s, during the V'ger incident, Commander Will Decker and Lieutenant Cleary did repairs on the pattern buffer but it malfunctioned while Cleary was replacing the transporter sensor. Janice Rand attempted to beam Commander Sonak and a female officer aboard the Enterprise but they were both killed. Scotty later repaired the pattern buffer and the sensor in time to beam Ilia and Leonard McCoy aboard. (Star Trek: The Motion Picture)
When weapons were beamed to a rebel camp on Krios from a cargo transporter of the Enterprise-D in 2367, Miles O'Brien asked Geordi La Forge if he could check the reliability of the pattern buffers. (TNG: "The Mind's Eye")