When Charlie is stabbing Caril-Ann's father, the knife becomes bloody after a few stabs. The camera turns to Caril-Ann's little sister for a while, and when it turns back to Charlie, the blood on the knife is gone.
After Starkweather kills a lady and her maid in their house, his white T-shirt is covered with blood. But when he comes downstairs after the second murder, the shirt is perfectly clean.
When Sheriff Merle Karnopp is waiting outside of a house for Starkweather to come out his partner has a shotgun drawn and pointed at the house. The gun is a solid rubber dummy-gun that prop people give to actors when they don't want to give them real guns. Usually these are only used for deep background or motion, not steady close-ups.
According to the screen captions, the Starkweather murders took place in Nebraska and Wyoming in late 1957 and early 1958, a time of brutal winter weather. Yet nobody is wearing heavy winter clothing, there is no breath condensation, the trees are green and leafy, and there are bright red flowers in full bloom. The movie was obviously filmed in the late spring or summer.
Toward the end of the movie when the Sheriff's Deputy in the marked patrol car recognizes Starkweather on the side of the road he turns his emergency lights on and makes a u-turn. Through the back window we see red and blue flashing police lights. In the 1950's police vehicles did not use blue emergency lights.
When Charlie shows Caril-Ann the money he took from his first victim, we see dollar bills that are Federal Reserve Notes with a visual design that was first introduced in 1963 - several years after the movie takes place. In 1958, the money would have been Silver Certificates, which had a very different look.
In the beginning, after killing the toad and Charlie's father comes by, a mic is clearly visible in top of the screen.