When walking through Snowdin, you see Sans near his hotdog stand and walk up to talk to him. But... you notice Sans wearing odd slippers. The slipper on his left foot looks like Alphys while the one on the right looks like Undyne. You mention this to Sans and he chuckles.

"huh? so you noticed them... I call this the 'shlippers'." Sans says, pointing his feet inwards so the slippers are kissing.

"You are my friend, so... If you want one they will cost you like $9,999."