I kinda noticed a pattern going on around here. It seems like Luffy loses his sandals before or during a big fight. Here's what I'm talking about:
-Arabasta arc: Crocodile dissolves Luffy's sandals with one of his attacks.
-Skypiea arc: Luffy's sandals get digested while in the python.
! -World Government arc: Luffy's Gear 3rd breaks through his sandals while battling Lucci.