And no man puts a bit of new cloth on an old garment, for by pulling away from the old, it makes a worse hole. And men do not put new wine into old wine-skins; or the skins will be burst and the wine will come out, and the skins are of no more use: But they put new wine into new wine-skins, and both will be preserved. Matthew 9:16-17
When new wine ferments, it produces gas and stretches the wineskin. New wineskins which are more flexible can still hold the wine, but old wineskins will burst. Similarly, new cloth is more flexible and therefore shrinks more when it is washed. If a piece of new cloth is patched onto an old garment, the garment will be torn and produces an even bigger hole.
Lord Jesus used these examples to illustrate that new behaviors under the New Covenant are more flexible, and must therefore be put under new doctrines. In the New Testament, disciples follow guidance of the Holy Spirit even though it may be contrary to the Old Testament Laws. For example, Jesus dines with tax collectors and sinners who were considered "unclean" (Mark 2:16-17), they "work" on Sabbath Days (Mark 3:2-5, 2:23-28), and His disciples also did not fast like the Pharisees (Mark 2:18-19). If people at that time had understood the necessity for this change, they would not have criticised Lord Jesus and His followers for not adhering to the inflexible old doctrines which places tremendous limitations on the new practice.