When does this dialogue come in?

[HappyHam]

People in the Undertale community refer to Sans as a "smiley trashbag." It is suggested that Flowey addresses him this way, but when in game does this actually happen? I have no knowledge of this line of dialogue at all. :/

[Peleg The Cynic]

In A Spasific Nuteral Ending

[Meepo]

I've never gotten it, but I know, if you spare Flowey, and if you meet some requirments, Flowey will address him as "Smiley Trashbag".

[(Edgy) Asriel Dreemurr]

You spare Flowey after the Photoshop Flowey Boss, then every time you reset and fight Asgore again, Flowey will kill him and just teleport you to the ending without fighting you. However, Flowey will have a bit of dialogue for the first few times you do this. The "smiley trashbag" line comes from one of these conversations.

[HappyHam]

Aha. Thank you, everybody!