In act 2 scene 3 -4 he cautions Romeo saying, "wiseley and slow; they stumble that fun fast".Then he violates his own admonition. HOW??
Juliet even said in the previous scene that their marriage would be too soon, too rash, and too unadvised.
The Friar's role of mentor reminds me of some of the young mentors our teens have today. The Friar rode a fine line between friend and advisor with Romeo. A good advisor will also keep in contact with a kid's parents and deal with problems when they arise even if it means betraying the kid's trust. This shows the Friar lacked confidence if he worked so hard for a kid's approval.
Even though he gave the advice, it was really up to Romeo to take it or not. Romeo certainly ended up stumbling because he went through everything so fast. The problem with being a teenager is the feeling that if I don't do it now and my way, it just might not happen. Romeo is at fault for the pace, the Friar is at fault for not getting the parents involved earlier even though he thought this would relieve the families of their feud.
Another inrepretation for the Friar's (and Romeo's) haste in the play 'Romeo and Juliet' by William Shakespeare is his anxiousness to prevent 'an occasion of sin.' For example, he can see and sense Romeo's haste, and that of Juliet and may feel that it is better for them to be married if they are going to spend the night together anyway. At least, in the eyes of the church/God, they are then married, one flesh, man and wife. The Friar may feel he is in the right doing God's work here, but nowadays parents, professionals and teachers would see this very differently. He makes the situation worse because he does not stop to think about other matches the parents may have in mind. Despite his reservations, he seems to get carried away himself.
You are talking about Friar Laurence here.
What he does that violates what he is saying is that he marries Romeo and Juliet. He does this the same day, not being cautious, not thinking about it, etc. He does this because he hopes that the marriage will get the Capulets and Montagues to stop fighting with one another.
You could say that he does this later in the play while trying to help Romeo and Juliet. He comes up with his crazy plan about the potion on the spur of the moment.
Neither of his hasty decisions turns out well.