In the fourth inning, during Harper's second plate appearance, Guillen felt he was disrespected by a gesture from Harper, and he went out to say a few words to Foster.
The incident created some excitement for a couple of minutes, but it didn't boil over any further as the game progressed.
"Ozzie complained that the pine tar was too high up on Harper's bat," Nationals manager Davey Johnson said. "So we changed it. Then, he was still chirping about it. It got on the umpire's nerves, it got on my nerves.
"He was trying to intimidate my player, I guess. He was hung up on our player. He does what he has to do. One of my pitchers complained one time, and it was the first time I saw him do it all year. Some strange things go on out there."
Guillen, who had repeatedly praised Harper for days when asked about the rookie, didn't get into specifics. He said he planned on speaking to Johnson about the 19-year-old outfielder.
Asked what he said to Harper, Guillen joked: "Nothing, I was just telling him how cute he was. Something happened the inning before, and I don't like it. I was talking with the umpire about it."
The fiery Marlins manager did his best to avoid stirring up any more controversy following Sunday's game.
But he did note that the incident involved Harper pointing to his bat, and mockingly examining it.
"Something like that," Guillen said. "But for the first time, it's going to stay between us. I could say a lot of [stuff] about this kid. I've been praising this kid. The last three times [the media] asked me about him, I said he is a great player.
"What he did today was unprofessional. I'm not going to tell you guys what he did. I'm not going to be talking about this guy on ESPN or Baseball Tonight or everywhere. I'm going to talk to his manager in a little while."
Harper, 0-for-4 on the day, went to a different bat after the pine-tar question was raised.
"He was just yelling," Harper said. "Yeah, I switched bats. But I just didn't feel comfortable with the first one, so I moved to the second one."
Harper viewed Guillen's action as a manager standing up for his team.
"That's the type of manager Ozzie is," Harper said. "He's a great manager to play for, and he's going to battle for you no matter what. That's a manager you want to play for."
Did the rookie feel Guillen was trying to intimidate him?
"Not sure," Harper said. "Like I said, he's a good manager, he's a fiery manager and he battles for his guys. That's what he does."
How high pine tar can be on a bat is an issue that has been going on for decades.
"It's such a fine line," Johnson said. "It's supposed to be at the top of the label, and some guys it might be over half an inch or something. There's still a foot of barrel to hit it with. If you hit it with pine tar, it's going to shatter everything. They replace the ball all the time, so what's the big deal?
"[Ozzie was yelling] at Harper. The umpire was trying to calm him down. But with Ozzie, it's the more the merrier."
The Nationals had an issue with pine tar on a pitcher's glove last month during their Interleague series with the Rays. Johnson had the umpires look at the glove of reliever Joel Peralta, and pine tar (a foreign substance) was inside.
Peralta was ejected, and Johnson and Tampa Bay manager Joe Maddon made headlines with their comments.
Marlins left fielder Logan Morrison felt Guillen took the high road to not have the umpires stop the game to check Harper's bat.
"Ozzie did it the right way," Morrison said. "He said, 'I don't want to make a big deal about it, and he told him to watch out about that pine tar.' I think Ozzie did it the right way, and then Harper, I guess, came up the next at-bat and maybe pointed at his bat. Ozzie felt he was kind of showing him up.
"[Ozzie] did him a favor by not going out there and saying, 'Hey, your pine tar is too high to the umpire.' I don't know if that's an ejection, or what. But he did it in a way that wouldn't show Harper up, and Harper showing him up was kind of a slap in the face, I guess."
Morrison pointed out that the Nationals raised the issue of pine tar on the glove of Peralta.
"Coming from what happened with the Rays thing, I think Ozzie took the high road there," Morrison said. "With the Rays, with Peralta having pine tar on his glove, they didn't seem to care. Ozzie definitely took the high road. I feel like he did it the right way.
"Harper maybe got a little emotional or whatever. It's part of the game. He plays hard, wears his emotions on his sleeve. I'd like to have him on my team, he's a good player. That's not necessarily a bad thing. You want guys on your team who have a chip on their shoulder."