Mark Redman's 10 runs allowed in the first inning in Los Angeles was the first time a pitcher had allowed that many in the first inning since last year. Jason Jennings of Houston coughed up 11 runs in the first to the Padres in a game.
What was unusual was that Redman came back out for the second. Actually, it was more unusual that he even survived the first inning as surrendering 10 runs in the first inning is a universal distress symbol to managers that the pitcher is having a bad game.
I tried going through the Baseball-reference.com Play Index to look for pitchers who might have given up 10 runs in the first and then kept pitching. I quickly determined that such a search would be fruitless. But you're welcome to double-check.
Tony Mullane of Boston did give up 16 runs in the first to Baltimore in the first game of a doubleheader on June 18, 1894, but I couldn't tell from the Sporting News boxscore if Mullane pitched further in the game. Boston won the game 24-7. Mullane most likely didn't finish the game and I think Bert Inks finished the game.
Baltimore still won the NL that year with an 89-39 record. Mullane made four more starts for the Orioles before he was traded to Cleveland for John Clarkson, a future Hall of Famer, who ended up not appearing in any games for the Orioles and was out of the majors after the season.
In most people's memories, the biggest first inning was Boston's 14-spot put up in the first against Florida on June 27, 2003. The Red Sox scored all those against three different Marlin pitchers, the first two of whom failed to retire a batter as the first 11 Boston batters reached safely.