John 3:35 The Father loveth the Son, and hath given all things into his hand.
Matthew 20:23 ...but to sit on my right hand, and on my left, is not mine to give, but it shall be given to them for whom it is prepared of my Father.
Critics citing this one should check linguistic options in a Greek lexicon. The aorist infinitive dounai (of didomi ) which the AV translates as "to give" may also be rendered "to grant". See the BAGD lexicon entry for this fact. Thus, if the best rendering of dounai is in fact "to grant" in the Matthean passage, then there is absolutely no argument for a contradiction here, for Jesus' words to the Sons of Thunder that "to sit on my right and my left is not mine to grant" merely is a statement about the exercise of Jesus' authority in this particular matter, which does not at all contradict John 3:35.
Those who don't appreciate this basic fact and wish to pursue the argument further must somehow demonstrate that the ability to grant the seating at His right and left is a "thing" in the same sense as John 3:35. Recalling that there is no direct word for "things" (usually a neuter article or demonstrative is used in lieu of a Greek word for "thing[s]"), I do not know how this could be done.
If one instead favors the translation of dounai as "to give" as the AV has it, the assertion rests on the mistaken premise that not being able to give something somehow necessitates that one does not have it in the first place. But this is in error. I have my life but I cannot give it to somebody (only in a figurative sense such as were I to tell some hypothetical bride-to-be this is a sense of commitment -- but the request to sit at Jesus' right and left was a physical concept).
I have my family heirlooms, but I am not really free to give them to anybody (the relatives would be angry). I have my degrees but cannot give my neighbor my doctorate. I have my (dream) job at DePaul but it is not mine to give it to somebody (which violates all of the protocol in university employment). We see that there are situations (and it is not hard to think of them) where we have something but do not have the full freedom (for various possible reasons as in the examples) to give them. So, if Jesus states that the request is not his to grant, it does not at all follow that John 3:35 is contradicted. These two passages are really very poor candidates for contradiction.
That leads to this passage:
Matt. 28:18 Then Jesus came to them and said, "All authority in heaven and on earth has been given to me."
This verse cannot be cited as contradictory. Those who do so confuse "power" (exousia) with "nature" - who sits on Jesus' right and left hand is not a matter of power or authority, but a matter of established order. One may as well speak of the "authority" to make two and two equal four.