But Hamilton and the Angels didn't quit. They stormed back for four runs in the ninth inning -- helped by Hamilton's run-scoring single -- to send the game into extra innings at Anaheim Stadium.
Then Hamilton sent everyone home in the 11th inning when he slugged the first pitch, a fastball, from Boston reliever Craig Breslow into the right-centerfield grandstand for a 9-7 Angels victory.
It was his first walk-off home run as an Angel and fourth of his career, ending a game that took four hours and 22 minutes.
Hamilton said his teammates "encourage you to go up there and kind of redeem yourself" after his fielding mishap. "We all make those mistakes at times. They understand that you got to do what you can to bounce back."
In the eighth inning, Boston's Dustin Pedroia singled and Daniel Nava hit a sinking liner to right field to Hamilton's left.
Hamilton tried to backhand the ball as he ran past it, fell down, got back up but still struggled to pick up the ball, and then finally threw it back to the infield.
Pedroia scored, Nava was credited with a double and Hamilton was given an error.
"I'm still not sure what happened," Hamilton said. "I don't know if I tripped or -- I went down to get the ball and if I stepped on my glove for a second or what, but it was pretty, you know, miserable."
Hamilton said that after the Angels cut Boston's lead to 7-6 in the ninth inning, he also worried that his error "could possibly be something that cost us the game. Fortunately it didn't."
Hamilton also hit a two-run homer Thursday that tied the game and led to an Angels win over the St. Louis Cardinals.
Angels Manager Mike Scioscia said that "in the batter's box, there seems to be a lot more confidence now" with Hamilton.
And despite struggling at the plate most of this season, and the fielding blunder earlier in Saturday's game, Scioscia said, "Josh has a great ability to keep looking forward."