Or HaChaim on Numbers 11:28/אור החיים על במדבר י״א:כ״ח

1 ויען יהושע ויאמר…אדני משה כלאם, Joshua reacted and said: "my lord Moses arrest them!" What gave Joshua the right to make a halachic decision in the presence of his teacher? Our sages in Eyruvin 63 have said that anyone who makes a halachic pronouncement in the presence of his teacher will die without leaving behind children. The Talmud quotes our verse as its source. Perhaps we can explain what Joshua did with the help of the Sifri on verse 26 where the Sifri claims that Eldod and Medod kept prophesying that Moses would die and Joshua would lead the Israelites into the Holy Land. When Joshua heard these words he resolved to take a stand in the presence of his teacher in order to demonstrate his displeasure with this prophecy. He believed that Eldod and Medod were not fit to prophesy and indeed spoke falsely. By taking a stand he showed that he was not interested in assuming the leadership role prophesied by Eldod and Medod. He may have meant either one of two things with his suggestion that Moses lock these people up. The confinement could be temporary until the matter came to trial, or he meant that they be banished from this world. Either possibility is in accord with the two views we have quoted as to whether Eldod and Medod belonged to the 70 elders who had been chosen or whether they were the two men who had drawn blanks.

2The wording אדני משה, "my lord Moses," suggests that Joshua expressed his desire for Moses to remain his lord, i.e. that he harboured no secret desire to supplant Moses. The apparently unnecessary word ויען means that he replied to the words (the content) that Eldod and Medod had prophesied. At any rate, Joshua was punished for having spoken out in this manner at that time, as, after all is said and done he violated the rule of giving a halachic opinion in the presence of his teacher. [There is no evidence in the Bible that Joshua left children behind when he died; compare Chronicles I 7,27. Ed.]