Problem with AES-128 AddRoundkey Step I'm trying to self-learn the AES-128 algorithm and ran into a snag while following these slides I found online: https://kavaliro.com/wp-content/uploads/2014/03/AES.pdf

For the sixth round in the example given, during the AddRoundkey step, the third word of the matrix after MixColumns (F0) is XOR-ed with the third word of the 6th RoundKey (B7). This apparently results in (77) which doesn't correlate with my calculation of 47. However, the process seems to proceed with this error and still gets the right ciphertext in the end (I put the ciphertext through an online decoder and got the original plaintext back).

I'm very confused as to what I am missing here?
You are not missing anything, the calculation is wrong. The calculation of the key schedule that is.

If you postulate the round calculations are correct, the round keys must be wrong, and in fact that byte of the round key is indeed wrong. Reversing the AddRoundKey calculation from the round example, the valid round key byte resulting in 0x77 is 0x87, not 0xB7.

Therefore the slide was just wrong, the values may have been entered by hand, and 8 looks like a B, so that is very likely.