I wonder why the phrase is "When you see it you will shit brix," and not "When you will see it you will shit brix."
Is the version with two will incorrect? What grammar rule says that you should not use will see in the above phrase?
Adding to Elendil's answer, "When" refers to "the point in time at which an action occurs".
And at that point of time, you are actually "seeing it", which is why the verb is in the present tense.
When/If (somethings happens) then (something else will happen)
if (something will happen) would be impossible to evaluate as a condition since the future is unknown.
When you refer to the future in a conditional clause, you usually use the simple present tense, therefore the first sentence is correct.
Oh, and it's spelled 'bricks', as in the plural of 'brick'.