With the outfielders pulled in on the infield dirt and the bases loaded in the tenth, Jhonny Peralta hit a slow-rolling snubber to right-fielder Mike Baxter, who was playing near second base in on the grass.
The Chicago outfielder fielded the roller, but his throw home was wide, allowing Peter Bourjos to score the winning run as St. Louis improved to a Major League-best 25 games over .500.
St. Louis (49-24) was baffled all night-long by Cubs starter Jake Arrieta, who was dominant, allowing just one run on three hits over seven sterling innings.