Mistakes made by the countries being criticised by a 14-year-old girl who had not done thorough research. 
All comments are based on opinions and should not be taken seriously.

British
The Naval base had modern facilities, large dry docks, defenses like airbases and gun positions. However, the facilities there were just waiting to rust as there were no strong fleet of warships. The gun positions all faced the south of Singapore, but defense at the North was neglected. The airforce of the British were out-of-date, unlike the "zero" fighters of the Japanese. Those lousy planes were also outnumbered.

The British left behind large stockpiles of fuel and ammunition for the enemy. That was just like supporting the enemy to invade your homeland.

The British sent ships out but were bombed by Japanese aircrafts. Everyone knows that aircrafts would definitely get the upperhand when fighting with ships.

The British did not have tanks because they thought they were unsuitable in jungle warfare. However, the Japanese tanks killed British soldiers in Jitra. The British had a lousy sense of jugement.

The British were stupid enough to not attack the Sultan of Johor's palace when they knew that the Japanese was inside.

The Japanese made the British think that they intended to attack from the northeast. So, when the Japanese troops attacked from the northwest, they lacked in supplies and troops. (I have no comments on this as it would have been easy to fall under this trick. But then again, couldn't they have distributed the soldiers evenly in case of emergency?)

The British commander made plans for the British solders to withdraw to protect the town if needs be. When the Percival issued secret plans, they were interpreted as a command to withdraw immediately. Phrasing can be important in English language. I wonder how many more mistakes can the British of those times make.

The British did not see through the Japanese. Counter-attacks were dismissed because there were too many problems. The Japanese had more problems than them yet they did not know. They should have realised that whoever went far out from thei counrty to fight, especially such a small country as Japan, would be facing a lot more problems than those rooted there, even if those rooted there had a limited supply (see one epic in the Chinese war theories).

(my comments: Good for the Japanese!)

Russians
The 'Dog mine'.They should have tried to get a tank of some other country to test it out before they carried it out. Testing is most important, especially on dogs, which are known to familliarise themselves with only one thing.

After the Czech incident, why did they trust Germany? *groan*

Japanese
No comments. But I feel that they should have stopped attacking once they know that their power was getting weaker.

America
It is mainly their own fault that Pearl Habor suffered so much. They should have been alert at the first sign of trouble.  yet the officer told the smaller officials not to worry about it. I hope they fired that one, if he was still left alive after the bombings.