Look at these two verses in the KJV:
Acts 7:45 (KJV) "Which also our fathers that came after brought in with Jesus into the possession of the Gentiles, whom God drave out before the face of our fathers, unto the days of David;"
Hebrews 4:8 (KJV) "For if Jesus had given them rest, then would he not afterward have spoken of another day."
Comparing those verses with other versions, we see the following:
Looking at the context of these two verses, it is easily seen that they are referring to when Joshua was leading the Israelites into the promised land. So why does the KJV have "Jesus" in these two verses while the other translations have "Joshua"?
A look at the underlying Greek in these verses shows that "Iesous" is the name under discussion here. Everywhere else in the NT, it is translated correctly as "Jesus", because that's who it is referring to. However, in these two verses (Acts 7:45 and Hebrews 4:8), the name is the same, but referring to Joshua of the Old Testament, not Jesus of the New.
The answer lies in the fact that the name "Joshua" in Hebrew ("Yahowshuwa" , Strong's #3091) is the same name as "Jesus" in Greek ("Iesous" , Strongs #2424).
The problem is that these two names are NOT equivalent in English. In Hebrew, there is one name "Yahowshuwa", and in Greek there is one name "Iesous". In English, we have two names, "Joshua" and "Jesus", so when the translators see "Iesous" in the Greek, they must choose which English name to use, since it can either mean "Jesus" or "Joshua". Context is the key, and the correct choice for Acts 7:45 and Hebrews 4:8 is "Joshua".
It's similar to the following silly example. Suppose we want to translate the sentences "Look at the elephant's long trunk. He uses it to spray water on himself." into another language. We do fine up to the word "trunk", because in English this single word can mean (among many other things) "an elephant's nose" or "a large case". In the language we want to translate this sentence into, suppose there are two entirely different words, one meaning "an elephant's nose", and the other meaning "a large case." Of course, the correct word to use in the translation, because of context, would be the one meaning "an elephant's nose", and it would be a translation error to instead use the word that means "a large case". The exact same thing is happening here when translating the Greek "Iesous" into English. In Acts 7:45 and Hebrews 4:8, the Greek name "Iesous" should be translated as "Joshua", not "Jesus".