A confused user, seeing the /help output could (and has) entered the pseudo-commands (meant as descriptions), and since they are in the command list, it attempts to run them -- and crashes.

jshell> /-<n>
Exception in thread "main" java.lang.IllegalStateException
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool$Command.lambda$new$0(JShellTool.java:1287)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.processCommand(JShellTool.java:1234)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.run(JShellTool.java:1160)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:929)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolBuilder.run(JShellToolBuilder.java:239)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider.main(JShellToolProvider.java:119)
/w/s/jdk10 1045>> ./build/linux-x86_64-normal-server-release/images/jdk/bin/jshell
| Welcome to JShell -- Version 10-internal
| For an introduction type: /help intro

jshell> /<id>
Exception in thread "main" java.lang.IllegalStateException
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool$Command.lambda$new$0(JShellTool.java:1287)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.processCommand(JShellTool.java:1234)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.run(JShellTool.java:1160)
        at jdk.jshell/jdk.internal.jshell.tool.JShellTool.start(JShellTool.java:929)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolBuilder.run(JShellToolBuilder.java:239)
        at jdk.jshell/jdk.internal.jshell.tool.JShellToolProvider.main(JShellToolProvider.java:119)