From which act/scene was:When Puck turns Bottom into an ass, and then puts the potion into the eyes of the wrong Athenian, and brags about his...

In Act II, Scene 2 of Shakespeare's A Midsummer
Night's Dream, Puck mistakes the Athenian on whom he is to drop the love
portion.  After searching through the forest, he finally espies one who wears "Weeds of
Athens"(2.2.71) and mistakenly anoints Lysander instead of
Demetrius.

In Act III, Scene 1 as the mechanicals rehearse
their play in the wood where Titania lies asleep.  After Bottom savours the flowers of
the scene, he is supposed to hear a voice and exit.  Puck remarks it is a strange
Pyramust than he has ever seen, he does follow Bottom. Then, when Bottom reappears, the
others scream and flee, for Bottom now has the head of a donkey. Then, in Scene 2 of
this same act, Puck enters another part of the wood with Oberon, boasting of his
pranks:

My
mistress with a monster is in love

Near to her close and
consecrated bower....

Made senseless things begin to do
them wrong;

....I led them on in this distracted
fear,

Titania waked, and
straightway loved an ass. (3.2.31-35)

Puck goes on to tell
Oberon that he anointed the Athenian.  However, when Hermia and Demetrius enter, Puck
claims that Demetrius is not the one whose hair he cut; Oberon then order Puck to "go
swifter than the wind" and find Helen of Athens. However, as he prepares to leave, Puck
overhears Hermia and Demetrius, who is pleading for "a lover's fee" (3.2.114)  Puck then
asks if Oberon wants to watch "What fools these mortals be.