Whenever I ALT+Tab to check a different window then tab back into the game, keys like ALT, CTRL, and SHIFT to get jammed as if they're being held down or rapidly pressed until I manually press them to unjam them.
This can be reproduced by binding an action to one of those modifier keys and testing that action—such as fishing (the character will immediately cast their fishing rod). The most obvious one is typing in the text box: normally CTRL+Backspace deletes entire words, but whenever one ALT+Tabs then tabs back into the game, the text box is stuck erasing entire words when one backspaces.
This may also have to do with the client registering input despite being in the background while another window is in the foreground. For example, right-clicking something in a different window will cause my character to turn in the direction of my cursor anyway.