Romanowski has been involved in numerous altercations with both teammates and opponents. In 1989, while with the 49ers, he brawled with offensive tackle Harris Barton after Barton had taken exception to a hit Romanowski had made on wide receiver Jerry Rice during a practice session. In 1995, while with the Eagles, he was ejected from a game — and subsequently fined $4,500 — for kicking Arizona Cardinals running back Larry Centers in the head. Three more incidents occurred during the 1997 season while he played for the Broncos: In the first, he was fined $20,000 after his hit on then-Carolina Panthers quarterback Kerry Collins in a preseason game resulted in Collins sustaining a broken jaw; in the second, Romanowski spat in the face of 49ers wide receiver J.J. Stokes in a regular-season game played in December on a Monday night; and in the third — in the AFC Championship game — Romanowski almost precipitated a full-scale brawl after he taunted Pittsburgh Steelers quarterback Kordell Stewart following an interception Stewart had thrown in the third quarter of the game, won by Denver. Two years later, while still with the Broncos, he was fined a total of $42,500 for three illegal hits plus a punch thrown at Kansas City Chiefs tight end Tony Gonzalez, and was also fined an undisclosed amount for throwing a football at Bryan Cox of the New York Jets, the ball hitting him in the crotch area.

In August of 2000, Romanowski was indicted by a grand jury in Denver for allegedly obtaining phentermine, a diet drug with stimulant properties, under false pretenses; he was later acquitted of this charge, but during the trial he made statements to the media many regarded as being racially insensitive. Another on-the-field incident, resulting in a $7,500 fine, followed in 2001. He was also linked to drug use in the book Game of Shadows.

On August 24, 2003, as a member of the Raiders, Romanowski punched backup tight end Marcus Williams in the face during a preseason scrimmage; Williams sustained a broken eye socket and was ultimately forced to retire, leading to Williams filing a civil lawsuit against Romanowski, seeking $3.8 million in damages. Following a three-week trial, a jury awarded Williams $340,000 in compensatory damages on March 22, 2005.