Fiend is Like Friend Without the 'R' - S1-E4
Continuity mistake: Mandy hits a baseball with one of Grim's bones, yet when we see Grim, there are none of his bones missing from his body.
A Grim Surprise / Beast and Barbarians - S1-E9
Continuity mistake: When Mandy is staring in awe at Billy and Grim's surprise, the flower print on her skirt is missing. (00:13:50)
Little Rock of Horror / Dream a Little Dream - S1-E13
Continuity mistake: In Dream a Little Dream when it shows Grim, Billy and Mandy eating the pizza, the box is empty. But when they wake up, there is one slice left.
A Grim Surprise / Beast and Barbarians - S1-E9
Continuity mistake: In "Beast and Barbarians" just before Grim presses the big red button, there is a box of donuts on the table beside him. In the next shot when the control area he is sitting at is blown up, the box has vanished. (00:20:35)
Grim vs. Mom / Tastes Like Chicken - S1-E7
Continuity mistake: When Billy finds Grim sun bathing in "Tastes Like Chicken", Grim yells at him. This causes Billy's hat to pop off, showing he's surprised, but when that happens he is bald. Normally, Billy has red hair.
Continuity mistake: Mandy is originally wearing a green scout dress with the badge sash. But when Grim suggests Aunt Callie's Best Cookies in the World, Mandy's dress is colored pink with the sash.
Grim vs. Mom / Tastes Like Chicken - S1-E7
Continuity mistake: Billy and Mandy are in night clothes during Grim and Billy's Mom's fight, but they're in regular clothes again after the combat.
Fiend is Like Friend Without the 'R' - S1-E4
Continuity mistake: When Billy is in the junk yard, his Frisbee is by his feet, but when the ground starts shaking, in one shot it is gone. In the next shot it's back at his feet again. (00:01:40)
Big Trouble in Billy's Basement / Tickle Me Mandy - S1-E12
Continuity mistake: Mandy's flower on her shirt disappears throughout.
