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'.
I challenge the premise, implied in the title, that English has a future tense. I won't pursue the argument per se because there are many articles on the subject by people more qualified than I am. Here's the first scholarly article that caught my eye.
For the purpose of my answer, I'm going to accept as an axiom that English does not have a future tense. If you accept that then there is no mixing of tenses in the phrase:
"When you see it you will shit bricks."
"When you see (present tense) it you will (present tense) shit (bare infinitive) bricks."
In a similar vein:
"If you saw (simple past) it you would (simple past of 'will') shit (bare infinitive) bricks."