Virtually all the sources I can find claim that the intervals between adjacent guitar strings in standard tuning (EADGBE) should be perfect fourths, except for one major third. However, since a perfect fourth is 4/3 and a major third is 5/4, this means the interval between the two E strings would be (4/3)**4 * (5/4) = 3.950... which is about 2% flat from the perfect interval, 4 (two perfect octaves).
Wikipedia has a table of "String frequencies of standard tuning" in Hz. If you do the math, you'll find that none of the intervals are actually exact. All the "perfect" fourths are sharp and the major third is also sharp. The really funny thing is they are all sharp by differing percentages. ~~It doesn't seem to be perfectly 12-TET either (semitone = 1.05946..).~~ Actually, maybe it is 12-TET within the given precision.
Is the "perfect" interval tuning just a simplification? Also, I realize this applies to all stringed instruments, not just the guitar.