The United States informed the Soviet Union yesterday that it had inadvertently failed to list 16 defective Pershing IA missile stages among the U.S. weapons to be eliminated under the recently signed Intermediate-Range Nuclear Forces (INF) Treaty, the State Department said last night.

A State Department spokesman said the Pershing missile components -- eight first stages, seven second stages and one first-stage inert trainer -- have been stored at Longhorn Army Ammunition Plant in Marshall, Tex., for more than 10 years.

"It was a goof . . . an honest mistake," said an administration official about U.S. failure to provide the information to Moscow before the treaty was signed at the White House Dec. 8.

The Texas plant, which is about 150 east of Dallas and about 40 miles west of Shreveport, La., was previously reported to the Soviets as one of the facilities where the medium-range Pershing missiles had been made. Despite the fact that Pershing production ceased nearly 10 years ago, Soviet inspectors are entitled to go there under terms of the treaty.

According to the State Department, the 16 missile stages were rejected at the time of their manufacture "because they had defects which could not be corrected." They had remained at the plant and evidently were overlooked in the inventory of U.S. weapons subject to destruction under the treaty.

Officials said they are still studying data about the Soviet missile inventory received from Moscow in connection with the treaty. In general, the officials said, the Soviet data is in line with previous U.S. intelligence estimates. A few cases in which discrepancies are suspected are being reviewed, the officials said.