Continuity mistake: When Mica and Lizzie are running from the walkers on the railroad tracks, they easily outrun them and gain several meters' distance in a few seconds. When they are running to the house, the walkers have suddenly caught up and are directly behind them.
Other mistake: Straight after Carol and Mika find some houses and are leading Tyreese, Lizzie and (supposedly) Judith to them, Judith is missing throughout the entire scene. No one appears to be holding her or even bothered by the fact that she isn't there. Then she's back again in the next scene.
Suggested correction: Judith was in the baby carrier on Tyreese's back.