It all happened about 30 miles into the 15th stage (there are a total of 21 stages) of the world's most prestigious cycling event, when the fan hit American rider Sepp Kuss, sending him careening to the ground. Then, cyclists behind Kuss fell when they ran into him/his bike.

Thankfully, none of the cyclists appeared to suffer serious injury -- just bumps and bruises.

Sepp talked to Cycling News about the accident, saying ... "There was a spectator leaning into the road, I guess. It just happened suddenly, and that’s part of the Tour, there are a lot of people. Ideally, that wouldn’t happen, but it’s the biggest bike race in the world and a lot of people don’t know exactly what’s going on."

The U.S. rider added ... "unfortunately, somebody wanted to get a selfie. I didn’t really see it coming."

After the stage, which Wout Poels won, a number of different riders and teams cautioned fans ... don't interfere.

"I’d like to tell the spectators to enjoy the race and be there to cheer for us without standing on the road or pouring beers on us," Jonas Vingegaard, the overall race leader told reporters.

“Please be careful, so that the party remains a party for the racers but equally for you. You don’t need a phone to create mind-blowing memories,” Team Confidis tweeted.

It's reminiscent of 2021 ... when a fan, like this weekend's incident, caused a massive pileup after their sign made contact with a rider.