The difference with this one was that it would alter the course of his hockey career.
Two summers ago, shortly after signing with the Ottawa Senators, Karlsson went out for a light skate with a buddy back home. No pads, just sticks and skates and a puck. During the skate, the friend stumbled in front of him, his skate slicing into Karlsson's right knee.
"Right away when it happened, I felt, not pain, it's hard to explain. I felt something," Karlsson said. "I looked down, saw a hole in my pants, thought this is not good."
Karlsson was dead-on. At the time it was bad, very bad. A cut tendon, to be precise.
Now, with the perspective of time and the rewards of hard work, Karlsson puts a positive spin on the injury. It changed the pace of his development and helped mold him into a top, young AHL defenseman, one who paces rookie blueliners with 37 points (5-32) for Binghamton.
"I've been thinking about that a lot. I think this is the best thing that could have happened to me, to be ready," he said. "I think so far that was one of the biggest turning points of my career."
The original plan had Karlsson, 23, a fourth-round pick by Ottawa in the 2003 draft, going to the Senators' camp two seasons ago as a healthy challenger for a spot as a depth player in the organization. He still went, but the injury scrambled his mission.
He spent much of the 2007 preseason rehabbing, and, naturally, never had a wisp of a chance to stick with the Senators. He was sent to Binghamton, where he opened up playing the first two games of the season. But Ottawa, noting Binghamton's tremendous depth on the blue line and the way Karlsson was struggling at the back of the class, told him to go home and get more experience in Sweden.
"I had no preparation whatsoever. I don’t want to be in and out of the lineup when I knew back home I could play. For me that (going home) was perfect," Karlsson said.
Karlsson played a full season in Sweden, including a chunk of time in the Elite League. The injury-forced detour gave him a chance to swallow regular minutes, round out his game and rejoin Binghamton this season with a flying head start.
"The first division in Sweden is a very tough league. For me to move up from the second division, you have to be more careful with the puck," Karlsson said. "I think I developed a lot."
And, from Binghamton's perspective, just in the nick of time.
The 6-foot-2, 220-pound Karlsson has to be on the short list of AHL players most valuable to their teams this season. Veteran Sens' defensemen Brendan Bell, Brian Lee, Matt Carkner and Geoff Waugh have all been in and out of the lineup because of injuries and/or recalls.
Karlsson has jumped right in to quarterback the top-ranked power play in the AHL. His nine-game assist streak at the end of February set a franchise record.
"He's got tremendous poise with the puck. He's got that ability to move across the line, keep his head up, create some options," said Binghamton head coach Curtis Hunt.
"I didn't expect myself to have (37) points at this time of the year," Karlsson said. "Our power play is No. 1 in the league right now. That's been a good thing for me to get points. I've been around really good players."
At times it's been an effort to keep up, and Karlsson is the first one to blow the whistle on himself. Hunt made him a healthy scratch against Norfolk on Feb. 21, even though at the time Karlsson was piling up the assists.
Karlsson admitted he was tired, and the sit-down was warranted because his defensive play had grown sloppy.
"Hockey isn't just about getting points. I hadn't been playing good. He said, 'Take a day off. It might be a good thing,’" Karlsson said.
"A big challenge for him is getting to the intensity level necessary to be an NHL player. Being his first year over here, the grind, it's taxing," Hunt said. "He has the skill set. I believe he could play at the next level."
A much more rested Karlsson returned with a goal in a win over Hershey on Feb. 25, although scoring doesn't figure to be his calling card. Karlsson played forward until he was about 14, when his coach pointed out he didn't have the speed to make much happen up front.
The coach moved Karlsson to defense, where he's been battling the skating issue ever since. Improvement has come gradually, although it didn't help when Binghamton bruiser Jeremy Yablonski broke the bungee cord that the Senators were using for resistance training. Karlsson and his teammates looked on in disbelief.
"He's too strong. He has too much power," Karlsson said. "He just took off. The whole thing exploded. We all started laughing. We didn't expect the thing to break."
Karlsson comes across as someone with a sense of maturity and a smell-the-roses outlook well beyond his young age, and a reason for that jumps into his arms every time he returns to his Binghamton residence. Karlsson and his wife, Therese, have an 18-month-old son, Liam.
"It's a lot of fun. I would say it actually helps your game," he said. "If you come home from a game that's really bad, you see a little kid running around and screaming for dad, you don't think about hockey any more. It makes it easier to get over a bad game."
Such reassurance comes with one small catch that Karlsson is learning to deal with by shrugging his shoulders. Although Karlsson said he's fine while skating, when he kneels down to play with his son the pressure on his right knee causes a little twinge of discomfort.
Considering how the injury swerved his career toward a much smoother transition, it's a tradeoff Karlsson will take.
"It's healed, but it's almost like I have a second knee cap under my knee," he said. "I think it's like scar tissue. It's always going to be there. I'm going to have to live with it. It's not a big deal."