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Sometime ago I came across the issue of invisible characters in Strings.  These can really cause confusion because they are invisible.
    String a = "Hello\u200e";
    String b = "Hello\u200f";
    System.out.println('\'' + a + "' and '" + b + "' are length "
                     + a.length() + " and " + b.length() 
                     + ", equals() is " + a.equals(b));

'Hello‎' and 'Hello‏' are length 6 and 6, equals() is false

Invisible identifiers Imagine my reaction to discovering you can use invisible characters in identifiers in Java :P.  These characters cannot appear at the start of identifiers in Java but can be anywhere else.

 System.out.println("String _\u200e = \"Hello \";");
    System.out.println("String _\u200f = \"World\";");
    System.out.println("String _\u200e\u200f = \" !!\";");

String _‎ = "Hello ";
String _‏ = "World";
String _‎‏ = " !!";

which when run prints

Hello World !!

So we have three identifiers which all appear the same because they have different invisible characters in their names !!

Amazingly this code compiles, runs and prints all the characters which can be in an identifier but not start them. The code contains \u202e which really messes with the display of the code
for (char c‮h = 0; c‮h < Character.MAX_VALUE; c‮h++)
        if (Character.isJavaIdentifierPart(c‮h) && !Character.isJavaIdentifierStart(c‮h))
            System.out.printf("%04x <%s>%n", (int) c‮h, "" + c‮h);
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