When you pick up an item in creative mode, and your hotbar is full, the item disappears from your inventory. However the items appear again once you open another countainer (ex. Chest)

To replicate glitch:
1 - In creative, with no free space in your hotbar, open any item tab and throw some items to the ground close to you.
2 - Pick those items up.(don't leave the tab screen)
3 - Now open your survival tab, the items you just picked up will refuse to display.

This glitch exists since 1.6, as far as I can tell.