TYPO3 Core

The description says "list of IDs", where it should be "query part for a list of IDs or a single ID"

If you don't dive into the code but just check the description while using the variable i.e. within a hooked method, this will lead to an error or unwanted behaviour.

Setting it to a single integer will just remove pid from the query giving you a result from all possible pages.
Setting it to a list of integers will trow an error message.

In our case it was the cause for a bug in grid elements, showing child elements still in their containers even though they where moved to another page.

Found it in 4.5.x, but still there in current master