The retractable roof has failed only once during an actual game since the stadium opened in 1999, when a computer communication problem led to a 54-minute rain delay during a 13-5 victory over the Rangers on July 23, 2000.
That remains the only rain delay in the Mariners' 34-year home history, since they previously played in the Kingdome.
Saturday's issue certainly didn't interrupt FanFest and the stadium maintenance crew still was able to move the main roof panel in place over the center of the field, covering most of the activities. But the other two panels remained stuck in place on the east side of the facility, so light rain fell on both sides of the covered area at times during the afternoon.
A total of 5,218 fans attended Saturday's event, almost identical to last year's first-day crowd of 5,290. FanFest concludes Sunday from 11 a.m. to 4 p.m., with Japanese pitcher Hisashi Iwakuma making his first appearance in Seattle.
Top pitching prospects Danny Hultzen, Taijuan Walker and James Paxton will be back for their second day at the event on Sunday, along with new catcher Jesus Montero and a number of returning Mariners veterans.
Scott Jenkins, the Mariners vice president of ballpark operations, said the roof problem was discovered Thursday night and will take until sometime next week to repair, as crews are working to jack up the roof to get at the broken wheel and axle.
"The good news is it's now, and not when we're playing baseball," Jenkins said. "If it happened during the season, it could be a problem. But we have time to deal with it."
The roof has 128 wheels on a rail system that allows the three huge panels to roll back and forth as needed. The taller second panel was able to be moved over the top of the stalled panel Saturday because its rails are outside of the broken panel's tracks.
But the entire roof can't be closed now until Jenkins' crew sets up a complex jacking system to raise the damaged area and fix the problem.
"We've done maintenance activities similar to this where we've replaced a wheel, but this one is in a tough spot," he said. "We were hoping to get it done without impacting FanFest, but tomorrow we'll have Panel 2 over the center of the field. We can't move Panels 1 or 3 right now until we get the wheel replaced and that won't be until next week sometime."
Jenkins said the roof has been "incredibly dependable" since the park opened and is closely inspected each year, but that the problem came in a place that is hard to see.
"We'll learn a few things from it and be better prepared and have more parts on hand," he said. "And just proactively, since this is a new kind of failure with the axle, we're going to do some ultrasound testing of all the axles just to make sure we don't have anything down the road we might have to deal with."