that

What bothers me about 'that?' I don't like it when sentient beings are referred to as 'that,' rather than as 'who.' Take that, CMS.

See also, who.

If, dubiously, 'that that' is grammatical as the beginning of a fused-head noun phrase (i.e. if we can say 'That that I said is true' to mean "That which I said is true" = "What I said is true") then we can construct a sentence with four consecutive uses of 'that':

It is evident that that that that nasty man said is true.

In truth, however, the sequence is illusory anyway, since the first and third uses are the subordinator 'that', while the second and fourth are the determinative, which is for all practical purposes a different word.