This is one of those things that just seems to be so stunningly obvious that you should have thought of it yourself every time you make a dense footprint (say 0.65mm BGA or less) … but I’m ashamed to say it never even crossed mine, despite having made several BGA and CSP footprints and boards.  Trying to make some tight BGA pads with a specific diameter?  0.3mm in Eagle may not turn out to be the 0.3mm you’re expecting on your PCB, and Andrew Zonenberg explains why in PCB Fab Characterization on his excellent blog Silicon Exposed.