What warning does Friar Laurence give Romeo foreshadowing future events of Romeo and Juliet?

Friar Lawrence warns Romeo, "Wisely and slow, they that
run fast stumble." He gives this warning after agreeing to marry Romeo and Juliet. He is
essentially warning against the hastiness of their union. They have only just met, and
he is cautioning them against moving too
quickly.

Ultimately, his warning foreshadows their doom.
Romeo's tendency to act without thinking first directly leads to the death of both
characters. The friar states:

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"These violent delights have violent ends. Which
in their triumph die, like fire and gunpowder, which as they kiss, consume."
(2.6.9-11).

The violent
delights, or the love, of Romeo and Juliet, often have violent ends, in this case death.
While their love may be beautiful and powerful, it is also brief and destructive. Thus,
with his words, the Friar unknowingly foreshadows the passionate but brief love of Romeo
and Juliet, which ends in death.