Why doesn't a go board have an 'I' ('i') column?
Zeiss isn't completely correct, this is actually the less common reason for omitting the I. The more common reason why I is often omitted where it would normally be expected between H and J is because in sans serif fonts the uppercase "I" is completely identical to the lowercase "l" (L). In a go board if you see a move noted in uppercase as I6 or lowercase as l6 without any other context you could not tell the difference in these fonts.

Confusion between a letter and a number would be much less common, since the letter in go is given first and the number is given second for hybrid notation, but confusion between L and I would be possible and is avoided by not including an I at all, thus guaranteeing that L and l both refer to the L column.

(for anyone with an interest in typography, this was actually intentional, allowing the lowercase l to be used in place of an uppercase I made it easier on the typesetters, they had more available of both that way and had less different symbols to worry about. Same way p and d or b and q are the same just rotated)