The polyfuse that is used has an "off factory" resistance of about 0.11 Ohm, and after soldering becomes something like 0.2 Ohm, (because it becomes hot from soldering). Tyco the manufacturer claims that after (repeated) blowing and cooling off (say recovering, as it can take days to "cool off") the typical end resistance will be near 0.45 Ohm.
a thought experiment.
Lets say that you indeed have a "bad" fuse, (with a 0.45 Ohm resistance) then what amount of current would be necessary to cause a 0.3 Volt drop?
It would be I = V/R or 0.3/0.45 = 660 mA, which indeed sounds reasonable!
What kind of current is needed for 0.3 V to drop across the polyfuse, while it is still "fresh", that would be I = V/R or 0.3/0.2 = 1.5A which is obviously way more than a PI would normally draw.
A few questions:
* Did you indeed power the PI with nothing else plugged in, only the SD-card?
* Did perhaps the SD-card became warm after your test?
* What is the open voltage of your power supply?
its possible that it will cause the over-voltage protection on your PI to trigger, which will cause a massive current draw, and then 1.5A is indeed quite possible!
The state of the package should not affect the state of the polyfuse, so that is a bit of an irrelevant comment, unless its meant as a general negative comment about the distributor
Unless you are unlucky enough to have received a "refurbished" PI that has a previously blow fuse, the logical explanation would be that something is drawing in excess of 1A, and that something would be something that both PI's share, the SD-card.
If you are extremely sure the PI had a blow fuse when you got it, I would return it again, and next time I would measure the resistance of the fuse before connecting anything! A resistance of almost half an ohm should be easily measurable.