Consider the following code:
---
public class T {
    void t() {
       Object o = null;
        switch (o) {
            default -> {
                switch (o) {
                    case String s -> System.out.println(s);
                    default -> {}
                }
            }
        }
    }
}
---

This fails to compile:
---
$ javac --enable-preview -source 17 T.java
Note: T.java uses preview features of Java SE 17.
Note: Recompile with -Xlint:preview for details.
An exception has occurred in the compiler (17-internal). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.AssertionError
        at jdk.compiler/com.sun.tools.javac.util.Assert.error(Assert.java:155)
        at jdk.compiler/com.sun.tools.javac.util.Assert.check(Assert.java:46)
        at jdk.compiler/com.sun.tools.javac.jvm.Gen.handleSwitch(Gen.java:1310)
        at jdk.compiler/com.sun.tools.javac.jvm.Gen.visitSwitch(Gen.java:1193)
        at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCSwitch.accept(JCTree.java:1294)
---