Simply playing the wrong videotape is one thing. But when the videotape is evidence, and it's played in the courtroom of Judge Thomas Penfield Jackson at the Microsoft antitrust trial, watch out for fireworks. That's what happened on Feb. 2, as one of the company's key exhibits was shown to have flaws that, if nothing else, suggested poor quality controls on Microsoft's part in preparing evidence.

At issue: a videotape Microsoft submitted into evidence that, according to Microsoft Senior Vice-President James Allchin, showed that a government's program designed to remove Microsoft's Internet Explorer from Windows 98 made Windows run slower and less efficiently. This, Allchin and Microsoft lawyers asserted, proved their point that Internet Explorer and Windows 98 aren't separate products at all, but are so intertwined that when the former is removed, overall performance is inhibited. Showing that Microsoft's Internet Explorer is a separate application is critical to the government's case in proving that the company violated antitrust laws.

But under cross-examination by Justice Dept. lawyer David Boies, Allchin couldn't say for sure that the slow computer shown in the videotape had the government program installed in it. In fact, Allchin conceded, the computer in the videotape seemed to be different than those used in the testing. "From what I see here, they filmed the wrong system," Allchin said.

GRAIN OF SALT. Boies was quick to pounce, accusing Microsoft of presenting faulty evidence. If Microsoft's own tests lack reliability, how is the court supposed to lend credence to other evidence that the company enters, Boies told reporters later. In effect, because the videotape contained admitted flaws, all other evidence should be taken with a grain of salt, he asserted. "The court has to evaluate the credibility of witnesses" and decide if Microsoft's evidence is unreliable, Boies said.

Later, outside the courtroom, Microsoft lawyers and public relations specialists launched a major damage-control offensive. Company spokesman Todd Nielson said the problem wasn't one of credibility or reliability, but of video production. He said what actually occurred in testing the government's program was that a number of "virgin" computers -- that is, brand new -- were set up in a controlled setting, and each of these computers exhibited slowdowns in performance after the government's program was run.

What was presented in court, Microsoft conceded, was a videotape of different computers than the ones Allchin and the company were basing their testimony on. Nielson said the computers on the tape were just "actors." If there were problems with these actors, that doesn't mean there were problems with the actual results. The results were sound.

Microsoft went so far as to offer to ship the computers that were used in the controlled setting to Washington, D.C., to show that the government's program was indeed flawed and that the government could not separate Internet Explorer from Windows 98 without hurting performance.

In the end, nothing Microsoft did seemed devious, and if given another chance, company officials confidently predicted their evidence would support their assertions. The problem, however, is that Microsoft may not get a second chance, and the last thing the company needed was for its own evidence to be used against it in such a damaging way. And if the videotape flub leaves Microsoft unable to convincingly prove that the government's program hurts Windows 98's performance, the company will have been dealt a serious blow to its defense.

By David G. McDonough Jr. in Washington