Mix Column of Rjindael
It is because the values being calculated are the byte representation of a polynomial in a finite field, and if the polynomial is too large to be in the field it must be reduced.

The reduction polynomial for Rijndael is $x^8 + x^4 + x^3 + x + 1$, which has a byte representation of 0x11b, and after you have shifted the byte by 1 bit, the 0x100 is no longer required so 0x1b or 27 is XOR'd against the value to complete the reduction.