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My opponent has two of my "birds of paradise"(0/1) under his control using a "control magic" on each one.
He also controls a "flying men"(1/1), a "serendib efreet"(3/4), several basic "islands", a "sol ring", "black Lotus" and "mox sapphire".

When I cast a "fireball" over "birds of paradise"(0/1), "birds of paradise"(0/1) and "flying men"(1/1), I select "6" mana for the X. I select these three creatures and forge request me to pay "R8" instead of "R6".

Note.- In the prompt I read x=6 when I select the three creatures.. but finally the casting cost requested for X is 8 (2 more)

I think you aren't understanding how fireball works. X is how much damage will be done split over all targets. The base cost is R + 1 per each target after the first. If you want to deal 1 damage to 3 targets it costs.. R + 3 + 2

ok ok, I know how fireball works... but I see the problem now.
By mistake I selected x=6 instead of x=5.. so forge though that I wanted to deal 2 damaged divided into 3 creatures.. so R+6+2.

Forge was ok. It requested R8

Nevermind.. Im sorry for the false warning