Saw this for the first time in many, many years recently (or possibly ever?) and while it's aged surprisingly well, I couldn't help but be reminded of the many, many mistakes that people have spotted in it! Here are a few of the best...
During the scene when Dorothy and Scarecrow are fighting with the trees, Scarecrow says "I'll show you how to get apples" and he gets hit by the apples. In the very next shot, a quick view of Dorothy reveals she is wearing black shoes, not her ruby slippers.
During the sequence where Dorothy meets the Scarecrow, Dorothy's pigtails are first short (above her shoulders) and as the song progresses her hair gets longer (below her shoulders), then short, and then long again.
In the scene where the Wizard is about to bestow 'awards', the Scarecrow is lined up second from the left as the Wizard begins to speak. In the next shot while the Wizard is still speaking, the Scarecrow 'jumps' to the far right.
At the start of the "lions and tigers and bears, oh my" sequence, Dorothy, the Scarecrow and Tin Man are all standing separately, then the shot cuts and they're suddenly arm in arm.
When trying to escape the Witch's castle, the big doors close, and the gang stands in a line and starts banging on the doors. Between shots Dorothy and the Tin-man switch sides.
As the chandelier falls on the guards at the witch's castle, all the candles are blown out by the time it lands. The next shot shows some of them relit.
After the Tin Man's dance, Dorothy and the Scarecrow shuffle over to a tree trunk where the Tin Man then sits. During this, the oil can in Dorothy's basket clearly falls out, but in the very next shot it is back in the basket.
At the end when the Wizard and Dorothy are in the balloon basket, the tie down ropes on the right side keep going from tied to untied and back to tied again through different shots.
Just before the Wicked Witch leaves Munchkinland she says to Dorothy, "just try to stay out of my way" and her broomstick is facing upwards. In the next shot it's facing downwards.
When the Lion is pulling up the Tin Man with his tail on the way to the castle, you can see a square block inside the Lion's costume to hold the tail on.
The Tin Man starts chopping the Witch's door and it's not the same one we just saw three shots before. The wood is a different color and the dark metal brace has been polished in the close-up.
Glinda tells Dorothy, "Their magic must be very powerful " and Dorothy has her left hand around Toto. In the next shot she's holding the flowers in front of her.
After the Tin man opens the door with his axe chopping its wood, Dorothy steps out and the door is totally fixed.
In the scene where the Wicked Witch in Munchkinland goes over to observe the ruby slippers on her dead sister's feet, when she is up close there's not much space between her and the slippers but when she turns around and says "they're gone," she's further away, then she's closer again in the next shot.