The target, in this case, was to drop a squirrel (dead I think) into a container.
But I had Control(Ender) pressed instead of Shift.
So It did not show up in the container ... but it also was not to be found anywhere else.
Don't think it was dead. But live ones generally don't run away immediately when dropped. And the squirrel icon was active, so not easy to miss if it was on the run.
(Not on this account)