Jason Jennings of the Astros gave up 11 runs (all earned) to the Padres at home today in the first inning. He was taken out with two outs and replaced by Mark McLemore.
This hadn't happened since ... last year on August 13, 2006 when Luke Hudson gave up 11 runs (10 earned) for the Royals against Cleveland.
There had been no prior instances in the last 50 years according to the Baseball-reference.com Play Index of any other pitching giving up more than 10 runs in the first inning of a start, although I'm certain it has happened before.
Make that 100% certain, the MLB record for runs allowed in an inning by a pitcher is 16 by Tony Mullane of Baltimore against Boston (NL) on June 18, 1894 in the first game of a doubleheader. That happened in the 1st inning. Boston won that game 24-7. Mullane was at the end of a fairly distinguished career.
The AL record for runs allowed in an inning is 13 by Lefty O'Doul of the Red Sox on July 7, 1923 in the sixth inning of the first game of a doubleheader against Cleveland. The Indians won 27-3. Stories have circulated that O'Doul had upset Red Sox manager Frank Chance and the manager left him out to take a beating. O'Doul went back to the minors in 1924 and came back as an outfielder in 1928 with the Giants and batted .349 in his career.
Hal Kelleher of the Phillies gave up 12 runs on May 5, 1938 in the 8th inning to the Cubs at Wrigley Field.