One of the perils of translating Harry Potter before the series is complete is the fact that the final outcome is unknown. Little clues and casual lines that seem insignificant at the time can prove to be of vital importance in later books. This makes life difficult for the translator because small slips or subtle differences of emphasis become very obvious as the story unfolds. Below is one instance of this.
In Owl Post Again (Book 3, Chapter 22), Harry tells Dumbledore that Professor Trelawney went very strange -- stranger than usual, even, rolling her eyes and speaking in a deep voice -- predicting that Voldemort's servant was going to set out to join him before midnight. Harry senses that she was actually making a real prediction.
Dumbledore knows that Trelawney is an old fraud, but Harry's story seems to have mildly impressed him. Dumbledore says:
"Do you know, Harry, I think she might have been. Who'd have thought it? That brings her total of real predictions up to two. I should offer her a pay rise..."
This brief statement tells us that Sybill Trelawney has made one real prediction before this. It is not until Book 5 that we learn what it is. In fact, it turns out to play a pivotal role in the entire book, which is ultimately about Voldemort's efforts to find out the prophecy's complete wording. The earlier prediction alluded to so casually turns out to be crucial to the plot.
Unfortunately, the Mainland Chinese version manages to get this little clue wrong due to a mistake in translation. Instead of saying that this 'brings her total of real predictions up to two', the translation runs:
"Nǐ zhīdào, Hālì, wǒ rènwéi yě xǔ shì. Shéi huì xiǎngdào zhè-jiàn shì ne? Zhè bǎ tā zhēnzhèng de yùyán nénglì tígāo dào èrjí shuǐpíng le. Wǒ yīnggāi gěi tā jiā gōngzī le......"
English (meaning translated literally):
"You know, Harry, I think it may be so. Who would think this? This raises her real prediction ability to second level. I should give her a rise in salary....."
So instead of revealing that Trelawney made another prediction in the past, Dumbledore simply says that she has risen to second level -- not even top level, still stuck at second rate!
This mistranslation is due to careless or faulty reading of the original English. As a slip-up, it would not be terribly serious in the ordinary scheme of things. It is the translator's misfortune that this just happens to be a vital clue to what transpires later.
(My thanks to the site visitor who pointed out this error in the middle of 2003.)