Chris Iannetta then bounced the ball to Castro, who threw wildly to second, allowing two more runs to score on error No. 3 to tie the game at 3.
"The first one, I thought, was a ball he didn't glove well," Cubs manager Mike Quade said. "The second two were throws -- the ball's wet and we've got to get the ball to the right place. He's going to learn to play in tougher fields than this, and maybe you have to take a little extra time.
"If you're in a hurry with a wet pig, you're in trouble. You take a minute and maybe you don't turn a double play and you get one. They don't have weather like this in the Dominican very often, so it's another learning experience. We have to get better. I've been saying that and it's got to happen."
It's the second time Castro has made three errors in one game, and first time this year. He also did so last May 10 against the Marlins in his first game at Wrigley Field. Castro had made his Major League debut three days earlier in Cincinnati.
The youngest player in the Major Leagues at 21, Castro didn't make any excuses.
"I caught the ball and it was kind of wet," Castro said. "I tried to make the play. It's one of those things."
"Those balls were really wet," Cubs second baseman Darwin Barney said. "The wind was blowing and the rain's coming down sideways. [Castro] made the plays after that and showed some maturity there."
This was the first time a Cubs player made three errors in a single inning since Jamie Navarro was charged with three in the third inning Aug. 18, 1996, against the Astros.
The fans who braved the nasty weather gave a sarcastic cheer when Castro fielded Esmil Rogers' grounder and cleanly threw him out for the second out in the second.
"We talk about learning experiences as far as dealing with the conditions or playing in Chicago or anything else," Quade said. "You hear [the boos], but he'll deal with it and he'll deal with it fine.
"Most of his things to me are experience and fundamentals. He's working like a son of a gun. It's unfortunate that it's all in one inning. You make a mistake here, you make a mistake there, you want to clean it up, but when they come in twos and threes, that's a tough deal.
"We have to continue to understand the offense is great and all the rest of it is good stuff, but if you can't catch it, you can't win."
Castro, in the three-hole for the fourth game, went 0-for-5 and is batting .357. He ranks among the National League leaders in batting average, hits, and multi-hit games.
"It's one of those things that happens," Tulowitzki said. "You kind of have to throw it away and start over tomorrow. He's pretty confident in his ability. I don't think it's going to affect him."