In Act II of Romeo and Juliet, what is an example of forshadowing of more sinister events in Act II? For example when Romeo and Juliet get married....

There are
many examples.  First, the Friar's opening monolgue is foreshadows bad things that can happen
from good. In talking about the qualities of the plants, he mentions that some plants might
smell good, but when tasted they can be poisonous.  He says, "For naught so vile
that on the earth doth live But to the earth some special good doth give; Nor aught so good but,
strain'd from that fair use, Revolts from true birth, stumbling on abuse: Virtue itself turns
vice, being misapplied." Act II, scene iii

At the end of the
scene, the Friar foreshadows the negative consequences of acting quickly.  He says:
"Wisely, and slowly. They stumble that run fast."   is rushing
forward to marry , but that increases the danger that their marriage will "stumble".

In Act II, scene iv,makes a joke about Romeo's love for Rosaline having
overpowered him. Mercution says: "Alas, poor Romeo, he is already dead! stabbed
with a white wench's black eye."  This joke hints that Romeo will die.  After
that, we learn  has challenged Romeo to a duel. Danger is pending.

These
scenes leave the reader feeling unsettled.  Although we should be happy about the marriage,
small words and small events (like Tybalt's challenge) keep interfering and making us doubt that
happiness can last.