In chapter 604 Oda slyly drew Luffy picking his nose and stick the booger on Usopp’s arm.
No one noticed until Oda himself (through his former editor Naito) revealed this gold in One Piece Knowledge King quiz event held in Japan last year.
Naito added that Oda drew the scenes while laughing uncontrollably and looked forward for readers to point it out in SBS. It kinda made him a little sad that it went unnoticed. Never change, Oda.