If you use three separate if statements instead of an if / else if / else chain, MULTIPLE branches can execute when only ONE should. The AP exam commonly tests this by asking what prints when conditions overlap.

// BUG: Both branches can run for the same score
if (score >= 60) {
    System.out.println("Pass");
}
if (score >= 90) {
    System.out.println("Excellent");
}

In Java, an else always matches the CLOSEST preceding if that doesn't already have an else. Indentation can be misleading — the compiler ignores it. Always use braces {} to make your intent explicit.

if (x > 0)
    if (x > 10)
        System.out.println("Big");
else  // This matches 'if (x > 10)', NOT 'if (x > 0)'
    System.out.println("Small");

In a chain like if (score >= 90) ... else if (score >= 80) ..., the higher range must come FIRST. If you put >= 60 first, a score of 95 would match that branch and never reach the A or B branches. Always order from most restrictive to least.

// BUG: score 95 prints 'C' because >= 60 matches first
if (score >= 60) {
    grade = "C";
}
else if (score >= 80) grade = "B";
else if (score >= 90) grade = "A";

If no condition in an if-else chain is true and there is no final else, no branch runs and variables may be left uninitialized. The compiler may flag this as a ‘variable might not have been initialized’ error on the AP exam.

String label;
if (n > 0) {
    label = "positive";
}
else if (n < 0) label = "negative";
// Missing else: label is uninitialized if n == 0
System.out.println(label); // Compile error possible