Due to the high acid content it can be... My grandmother always did, and we thrived. If there was whipped cream already on top of it, I'd scrape it off and use fresh.
(DISCLAIMER: It is, of course, always better to refrigerate)
Can Key Lime Pie Be Left Out Over Night?
If I ate a bite of key-lime pie unrefrigerated for six hours will I get sick?
Yes, but it may not be as good as it would be if it were kept in the refrigerator. It wouldn't hurt the pie, though.