There are just too much of them.
English. Hopefully self-explanatory. Who on earth made irregular verbs and overloaded the words “get”, “set”, “make”, “put”, and several others to the ridiculous point of 50 meanings. On a tangential note, who made the lack of singular unknown-gender pronoun. (Or is it “it” for humans of unknown gender?) Oh wait, there’s also this stupid genders of words. As in “player” is usually female (I think; if false, just change accordingly), so if you call “That player is so smart that [pronoun] won ten times in a row”, you’re supposed to use “she”.
Time. Also hopefully self-explanatory. 365 days aren’t supposed to be divided to 12 months, and leap seconds and 29 February shouldn’t exist at all. If I’m used to it, I might say that today is 2014-269. (Year 2014 day 269; remember that I remove leap days so this is 488 days after 26 May. If I do my computations correctly.) Also, to a lesser extent, the number of hours in a day and the number of minutes in an hour should be equal, so that computing times is as easy as handling a large base for your numbers.
I may edit this later. Meanwhile, I need to have some decent sleep soon for a test for a scholarship tomorrow in Jakarta, and I should get up at 2. In the morning. Derp.
On another very unrelated note, no one has found the sunken treasure yet. Added a hint.