The following code or similar

        VaList vaList = VaList.make(builder -> builder.addVarg(ValueLayout.JAVA_LONG, 1L), MemorySession.openConfined());
        for (int i = 0; i < 10000; i ++) {
            System.out.println("nextVarg number " + i);
            vaList.nextVarg(ValueLayout.JAVA_LONG);
        }

leads to

nextVarg number 0
nextVarg number 1
nextVarg number 2
nextVarg number 3
nextVarg number 4
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length -7
at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:100)
at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:124)
at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:448)
at java.base/java.util.Objects.checkIndex(Objects.java:461)
at java.base/jdk.internal.foreign.AbstractMemorySegmentImpl.checkBounds(AbstractMemorySegmentImpl.java:397)
at java.base/jdk.internal.foreign.AbstractMemorySegmentImpl.checkAccess(AbstractMemorySegmentImpl.java:355)
at java.base/java.lang.invoke.VarHandleSegmentAsLongs.checkAddress(VarHandleSegmentAsLongs.java:81)
at java.base/java.lang.invoke.VarHandleSegmentAsLongs.get(VarHandleSegmentAsLongs.java:108)
at java.base/jdk.internal.foreign.abi.aarch64.macos.MacOsAArch64VaList.read(MacOsAArch64VaList.java:121)
at java.base/jdk.internal.foreign.abi.aarch64.macos.MacOsAArch64VaList.read(MacOsAArch64VaList.java:94)
at java.base/jdk.internal.foreign.abi.aarch64.macos.MacOsAArch64VaList.nextVarg(MacOsAArch64VaList.java:74)

On MacOS AArch64 or even to

openjdk 19-ea 2022-09-20
OpenJDK Runtime Environment (build 19-ea+28-2110)
OpenJDK 64-Bit Server VM (build 19-ea+28-2110, mixed mode, sharing)
nextVarg number 0
nextVarg number 1
nextVarg number 2
nextVarg number 3
nextVarg number 4
nextVarg number 5
nextVarg number 6
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00007f7ac193094b, pid=2247746, tid=2247750
#
# JRE version: OpenJDK Runtime Environment (19.0+28) (build 19-ea+28-2110)
# Java VM: OpenJDK 64-Bit Server VM (19-ea+28-2110, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0xe6e94b] Unsafe_GetLong+0x8b
#
# Core dump will be written. Default location: Core dumps may be processed with "/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %e" (or dumping to ..... core.2247746)
#
# An error report file with more information is saved as:
# ..... hs_err_pid2247746.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#

On Linux X64

It looks like at least the specification for
https://download.java.net/java/early_access/jdk19/docs/api/java.base/java/lang/foreign/VaList.html
or the #nextVarg methods deserves improvement and clarification - for example mention possibility of IndexOutOfBoundsException