If you collect from something and the drop is something you're maxed on you get a message (pop-up) saying 'you can't hold more XXX' (for example Fuzzy Moss)
The pop-up appears in the right upper corner and is black, it doesn't stay long, so you have to pay attention not to miss it. We call the message 'black flag' here on the forum.
You only get 1 black flag message, even if there were more items you couldn't receive.