as i found this glitch when i was using banners the glitch only works with stack able items it might not work first try the items have to be the same as each other you do need a full inventory to start with

so the first step is have an item in the 7th slot of your hot bar
and another in the 25th slot in your inventory counting from closest to the chest slot

you drop both of the items when in your inventory wait for them to turn into a stack then pick them up

then there should be one more

this glitch only worked for me with a resource pack
i have only tried this glitch in creative