When you press a key that opens a GUI, like 'e' for inventory,

and that GUI opens with a text box, like the chat box, or creative search box.

the key you pressed to open that gui will sometimes be typed into that textbox.

E.g. when pressing t for chat, sometimes t will appear in the textbox,
or pressing '/' for command input will sometimes double up to make "//" appear.
or, with creative with search tab selected, pressing e for inventory, sometimes the e will appear in the textbox

It's not a "ghost" character, the inserted character acts just as if you typed it.
It is not consistent.

Only tested the Creative inventory and the chat/command box.

happened with both a PS/2 Mechanical keyboard, and a USB Slim keyboard

This did not affect 1.12(.2)
This did affect some of the 1.13 snapshots