Massive election-day computer foul-ups and possible "human errors" threw the outcome of Virginia's U.S. Senate election into doubt yesterday.

Despite an unofficial count that showed Democrat Andrew P. Miller dropped further behind the apparent winner, Republican John W. Warner, election officials did not rule out the possibility of a Miller victory.

"I'm really not sure what the outcome is," Joan S. Mahan, secretary of the Virginia Board of Elections, said. "There is always the possibility of error, given the human factor."

Unofficial returns announced yesterday afternoon by the News Election Service showed that Warner's narrow lead over Miller had climbed to 4,512 votes out of 1.2 million in the closest general election in recent Virginia history.

The unofficial vote totals were 613,294 for Warner and 608,782 for Miller, a margin of about one-third of 1 percent.

But the impact of those figures was clouded by the uncertainty over their accuracy. The News Election Service, a consortium of news-gathering agencies and broadcast networks, was listing more precincts than Virginia election officials said the state has.

One point that made for uncertainty with the unofficial results was the question of how many absentee ballots had been counted. Some localities, such as Fairfax County, make a point of including those votes with their election-night results, but many others do not.

Troubled by such discrepancies, state election officials yesterday called a news conference at the State Capitol to disclaim responsibility for the confusion and to caution that official surveys of past election frequently have shown substantial differences from the previously reported unofficial results.

The errors have been so great that Miller easily could emerge as victor in the race when the state completes its canvass Nov. 27, state officials said.

Gov. John N. Dalton, for example, showed a loss of 7,000 votes from unofficial figures when state officials last year compiled the results of his landslide victory over Democrat Henry E. Howell.

The major problem with the Virginia returns appeared to have been a computer in New York operated by the News Election Service that was processing Virginia returns. It failed in the middle of the vote count Tuesday night, forcing wire service officials to rely on a backup system in Richmond.

Throughout the night and into yesterday, NES officials were reporting Virginia results with varying numbers of precincts - a point that mystified Virginia election officials, who pointed out that the state has 1,857 precincts.

In New York, Dick Eimers, executive director of NES, acknowledged computer failures during the election but said he knew of no errors in the results the service was producing.

In Richmond, however, there was mass confusion over the results. "Things have gotten out of hand . . . There are so many discrepancies," said Ed Young, an Associated Press news editor.

Both Miller and Warner forces yesterday were viewing the results apprehensively and shaping plans to monitor what seemed certain to be a lengthy recounting.

Warner press secretary Bill Kling said Warner, who had refused to claim victory, was "relatively comfortable" with the 3,000-odd vote margin NES has given the GOP candidate early yesterday. The Republicans would "settle at this point for 50 percent plus one vote," Kling said.

But he said campaign counsel Tim Ryan was checking recount possibilities and that the Warner organization throughout the state had been alerted to have supports present when local electoral boards begin to review the vote in their jurisdictions. That process will begin today in most communities.

A spokesman for the Miller campaign said its staff members were weighing options and discussing ways to raise the approximately $120,000 a recount might cost. Miller, a former state attorney general, refused to concede defeat and said he would await the state canvas.

Neither Warner nor Miller made any public appearances yesterday.

Warner planned to spend several days at his Middleburg farm with his wife, actress Elizabeth Taylor, who is recuperating from a throat injury, an aide said. Miller was reported resting in Richmond.

State law permits any candidates to request a recount if the winner's margin in the officially certified vote canvass in less than 1 percent of the total vote - in this case roughly 12,000 votes. The law provides that the candidate asking for the recount must pay its cost if the recount shows him the loser.

Should he win, the cost is borne by the localities in question, which in a statewide recount, would be every one in the state.

If the Miller-Warner election is the closest general election contest in memory, it is not the closest election per se. In the 1966 Democratic primary, William B. Spong Jr. defeated incumbent Sen. A. Willis Robertson by 611 votes out of some 433,000 cast.

Robertson, however, did not ask for a recount. In those days, allegations of vote fraud were common and wide-spread in Virginia elections, particularly in the mountainous Southwest where, it was alleged, bogus absentee ballots were often cast.

In recent years, however, Virginia elections have been noticeably free of alleged irregularities - partly due to general election law changes, the increased use of voting machines, and the administration of Mahan, a Republican appointee widely respected by legislators of both parties.

On Tuesday, despite the size and closeness of the vote, she said her office received not a single complaint of voting irregularity anywhere in the state.