The fatal mistake made by Russia's provisional government was

-They raised food prices to cover the cost of the revolution.

-They continued to participate in WWI.

-They murdered the Czar and his family.

-They invaded Crimea.

They continued to participate in WWI

Explanation:

The Russian provisional government was formed in 2nd March after the abdication of the Tsar Nicholas II, the last emperor of Russia. After the provisional government was formed, series of attempt was made to get the country together in the midst of growing anarchy but it seems not to be working.

The fatal mistake they made which led to more crises was the annoncement by the minister of Foreign Affairs Pavel Milyukov, who promised to continue the war to its glourious conclusion. This led to massive demonstrations by the workers and the soldiers due to their disposition of the war not being continued.