Continuity mistake: When Roy Till and Ignacio are bound to the bed by Nancy, the black ties around Ignacio's left wrist keep changing places between shots.
Continuity mistake: When Silas is painting the wall green, some parts jump from being painted to unpainted between shots.
Continuity mistake: When Andy walks towards Gayle to give him the Claritin, he steps over the sign that Gayle was holding moments earlier, only for it to appear in Gayle's hand in the very next shot.
Continuity mistake: When Audra is talking to Andy outside of the clinic, everything about her cigarette changes between shots (position, smoke, etc).
Continuity mistake: When Shane is on the couch eating, his spoon goes from beside the bowl to inside the bowl when the shot changes.
Answer: It's not uncommon for TV shows to suddenly "age up" baby/toddler characters in order for them to have more narrative purpose. Other examples of shows I can think of that did this are "Step by Step," "Last Man Standing," "Boy Meets World," and possibly "Dexter."
Phaneron