An excess build-up of EM residue in the exhaust ports can cause restricted plasma exhaust flow. The ignition of plasma exhaust will not cause damage to a starship's nacelles as long as the vessel has its hull plating polarized, and is traveling at half-impulse.
In 2151, the plasma exhaust flow became restricted aboard Enterprise NX-01, causing several system malfunctions. After investigating the phenomenon, it was discovered that Enterprise had a distorted wake pattern, which indicated that plasma exhaust was flowing too close to the ship.
In effort to discover the cause of the distortion, the crew ignited the plasma exhaust, revealing a cloaked Xyrillian starship. It was further revealed by the Xyrillian captain, Trena'L, that by sharing the warp field with Enterprise, their ship was able to use Enterprise's plasma exhaust to replenish their teraphasic coils. (ENT: "Unexpected")
The starship's need to expel plasma exhaust proved to be the Achilles' heel for the prototype Klingon Bird-of-Prey that could fire its weapons while cloaked as the Enterprise-A's photon torpedoes sensors were quickly adapted to follow the plasma exhaust trail of the Bird-of-Prey. (Star Trek VI: The Undiscovered Country)