Consider this source code:
---
public class InvalidEscapes {
    private String s = """
            \!
            """;
}
---

When compiled:
---
$ javac --enable-preview -source 14 InvalidEscape.java
InvalidEscape.java:3: error: illegal escape character
            \!
             ^
Note: InvalidEscape.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
1 error
printing javac parameters to: <path>/javac.20190712_182002.args
---

Note the "printing javac parameters" - this means there was an exception inside javac. Recompiling with -XDdev shows the exception:
---
$ javac --enable-preview -source 14 -XDdev InvalidEscape.java
InvalidEscape.java:3: error: illegal escape character
            \!
             ^
Note: InvalidEscape.java uses preview language features.
Note: Recompile with -Xlint:preview for details.
1 error
An exception has occurred in the compiler (14-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.RuntimeException: java.lang.reflect.InvocationTargetException
        at jdk.compiler/com.sun.tools.javac.parser.JavaTokenizer$TextBlockSupport.translateEscapes(JavaTokenizer.java:395)
        at jdk.compiler/com.sun.tools.javac.parser.JavaTokenizer.readToken(JavaTokenizer.java:1042)
        at jdk.compiler/com.sun.tools.javac.parser.Scanner.nextToken(Scanner.java:115)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.nextToken(JavacParser.java:263)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.variableDeclaratorRest(JavacParser.java:3249)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.variableDeclaratorsRest(JavacParser.java:3220)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.classOrInterfaceBodyDeclaration(JavacParser.java:3952)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.classOrInterfaceBody(JavacParser.java:3848)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.classDeclaration(JavacParser.java:3687)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.classOrInterfaceOrEnumDeclaration(JavacParser.java:3641)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.typeDeclaration(JavacParser.java:3630)
        at jdk.compiler/com.sun.tools.javac.parser.JavacParser.parseCompilationUnit(JavacParser.java:3474)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.parse(JavaCompiler.java:639)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.parse(JavaCompiler.java:676)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.parseFiles(JavaCompiler.java:1031)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.parseFiles(JavaCompiler.java:1018)
        at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:938)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:318)
        at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
        at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:57)
        at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:43)
Caused by: java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:567)
        at jdk.compiler/com.sun.tools.javac.parser.JavaTokenizer$TextBlockSupport.translateEscapes(JavaTokenizer.java:393)
        ... 20 more
Caused by: java.lang.IllegalArgumentException: Invalid escape sequence: \! \\u0021
        at java.base/java.lang.String.translateEscapes(String.java:3145)
        ... 25 more
printing javac parameters to: <path>/javac.20190712_181859.args
---