Through the 1940s Barks persisted to place the nephews' names in the wrong progression as seen on the poster. The normal progression is Huey, Dewey, and Louie.
Another example can be found in a note from the nephews to Donald in WDCS095, page 3, panel 6.

We all know that the ducks sometimes are shown with teeth or very long tongues inside their beaks. In this example Donald has both, which makes you wonder how his tongue can penetrate his clenched teeth!

When an artist is composing a panel he will sometimes find himself cornered, because he realizes too late that a certain gimmick will be lost unless he bends the rules a little. In this square egg story Barks wanted to show the book's title, The Egg and Us, but he was forced to place it on the backside of the book.

Quite often the ducks are seen speaking (in the shape of speech bubbles), although their beaks are closed. This sort of 'ventriloquism' may not be a grave mistake, but it is a bit disturbing to the reader. In this example the nephew on the right is 'speaking' through a closed beak.

Gladstone has a few facial features that differ from Donald's. One is that his eyes do not touch his beak. But in this panel Barks happened to portray Donald with the Gladstone look...
Another characteristic difference is that Gladstone's beak does not have a rim in the front. A few times Barks unwittingly drew Donald with the goose's beak.

In this story Donald and Gladstone are battling it out in an attempt to decide who is going to get a role in a theater play. Observe that the two metal swords are actually bending on impact. Although quite unrealistic, Barks might have decided to use the gimmick to underline the force of the blows.

When Donald is very surprised, enraged or commanding, Barks would sometimes place his beak at an impossible angle - in this case Donald's lower beak seems to be far to the left of his neck.
Another example can be found in FC0223, page 3, panel 4.

Barks always tried to show any landscape as accurate as possible, but in some instances he obviously forgot to check his backgrounds thoroughly. Here we are shown mesas, that should have been drawn with horizontal tops.

In this sports story we are presented with Barks' rendering of the Olympic flag - 5 circles in a linked row. The correct image consists of 3 interlocking circles accompanied by 2 circles in the next row. Barks continued to draw the wrong symbol until the last time it is shown; then the image is suddenly correct - but on a triangular flag!

It would seem that Barks miscalculated the space available when he drew this panel, because in order to get all three nephews in the picture he had to squeeze the farmost nephew's head.

Here Donald has been drawn with a left hand that defies normal physiques. It is quite impossible to hold your hand in the position shown. (The thumb should have been placed on the other side of the hand!
Barks made the same physical mistake when drawing neighbour Jones in WDCS289, page 4, panel 6.

In the late 1950s Barks changed Donald's torso to look a little more pear-shaped with a 'heavier' lower body. But in this panel Donald's proportions are totally wrong; too small head, too long body, too short legs...

The ducks seem to have 'loose' tongues depending on what they are doing. Mostly the tongues start back in their throats, but in a few instances they start in the middle of the beak for no apparent reason.
Another example can be found in WDCS218 (see the above drawing).

The JWs caps have tails emerging from the middle of the top in the first page of this story (the JWs own term for this is Hightails). In the rest of the story the tails are hanging from the back of the cap's tops.
Barks sometimes forgot how his characters were dressed; in WDCS202 Donald is wearing a medicine man's waistcoat that alternately has one, two or three buttons!

Look at Donald's left hand; it appears quite stunted. He is clearly missing his thumb, and he also seems to have an amputated finger...