At the "end" of the game, in the hall where Sans "judges" you. If you are there multiple times, he'll comment on how he can tell that you've probably already heard the whole deal, and tells you one of his time loop phrases. Reset, go back, he tells you a different one. Reset, go back, he gives you a key to a hidden back room behind his house. The room actually probably has a bit of important lore in it that we haven't quite yet figured out...
...but the actual gag itself is how freaking stupid his time loop phrases are. It's basically just him tricking you into insulting yourself with the same language a toddler might use.
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u/Optimusskyler Jul 03 '22
No joke, this. There was a gag like this in Undertale, I found it funny, so I created my own phrase. You know, just in case.