@Gruyere 

I added another "broSet:={}" just before the annoying loop, aaaaaaaaaand it worked. I really don't understand why, but thank you a lot for your comment :)

@Carl Love 

It's strange because I initialized broSet. Here is the entire code (not that big). 

> restart; with(plots);
> param := Matrix(%id = 109648200);
> brotSet = {};
> param[1, 1] := -.15;
> param[2, 1] := .85;
> for i from 2 to 100 do param[1, i] := param[1, i-1]+(1/100)*(.15+0.8e-1) end do;
> for i from 2 to 100 do param[2, i] := param[2, i-1]+(1/100)*(.9-.85) end do;
> for i to 100 do for j to 100 do a := 0; b := 0; for k to 160 do a2 := a; b2 := b; a := a2*a2-b2*b2+param[1, j]; b := 2*a2*b2+param[2, i] end do; if a < 2.0 then brotSet := `union`(brotSet, {[param[1, j], param[2, i]]}) end if end do end do;
Error, recursive assignment
> NULL;
> pointplot(brotSet, color = red);
Error, (in plots:-pointplot) points are not in the correct format
author: MONET Mikaël
Error, missing operator or `;`

The mistery is that it worked yesterday. I uninstalled and reinstalled Maple, but no, bugg still here. I checked the programm a million times è_é.

Thank you for your time