The parable at the beginning of Matthew 20 is about a landowner hiring workers for his vineyard. It's a parable about how the landowner treats workers who come late in the day. The late comers are at a disadvantage because they get to the vineyard after everyone else, and thus they can't possibly earn what the early workers earn. It is impossible, but the landowner gives them a full day's pay anyway.
The point of the parable is not that the ones who worked the longest didn't get what they deserved. The point of the parable is that those who worked for a short time got more than what they deserved.
I've read this parable a dozen times and thought, "That's not fair! Those poor men who worked all day must feel cheated!" But that's only because I believed that those who worked all day earned their wages fair and square. And those who worked for only one hour didn't deserve what they got.
But the kingdom of heaven isn't like that. God gives everlasting life to all: those who believe in Christ from infancy and those who believe in Christ on their deathbeds, those who are born of respectable parents and those born of jailbirds. All are paid the same because of God's generosity. Not because they deserve what they get. We all actually deserve damnation.
The trap that Christ was trying to show his disciples was believing we deserve God's generosity more than others or expecting God to treat us with some sort of special attention because we've followed Him longer or held a special position in a church or haven't been as terrible as somebody else.
If we walk into heaven and see Stephen Hawkings or Harold Camping or that cheat that ran off with someone's wife or the black sheep of our own family, and we say, "You don't deserve to be here!" then we falsely believe that we do deserve to be there. And that is to misunderstand grace entirely.
It is just as remarkable that God gives grace to me as he gives to others.