Misconception: Looking at the wrong digit
The most common mistake is looking at the wrong place value. For rounding to the nearest thousand, you only need to consider the hundreds digit. The tens and ones digits are irrelevant to the decision of rounding up or down.
For example, in 7,499, the hundreds digit is 4. Even though 99 is very close to 100, the rule dictates that you round down based on the 4. The correct rounded value is 7,000, not 8,000.