when compiling gcc-4.5.1 with fortify enabled, we see the warnings:

In file included from /usr/include/string.h:642:0,
                 from libjava/java/lang/natClass.cc:15:

In function ‘void* memcpy(void*, const void*, size_t)’,
    inlined from ‘java::lang::Object*
parseAnnotationElement(java::lang::Class*, _Jv_Constants*, unsigned char*&,
    unsigned char*)’ at libjava/java/lang/natClass.cc:1168:60:
/usr/include/bits/string3.h:52:71: warning: call to void*
__builtin___memcpy_chk(void*, const void*, long unsigned int, long unsigned int) will always overflow destination buffer

In function ‘void* memcpy(void*, const void*, size_t)’,
    inlined from ‘java::lang::Object*
parseAnnotationElement(java::lang::Class*, _Jv_Constants*, unsigned char*&,
    unsigned char*)’ at libjava/java/lang/natClass.cc:1184:60:
/usr/include/bits/string3.h:52:71: warning: call to void*
__builtin___memcpy_chk(void*, const void*, long unsigned int, long unsigned int) will always overflow destination buffer

if we look at the code in question:
...
  case 'D':
    {
      int cindex = read_u2 (bytes, last);
      check_constant (pool, cindex, JV_CONSTANT_Double);
      _Jv_word2 word;
      memcpy (&word, &pool->data[cindex], 2 * sizeof (_Jv_word));
      result = Double::valueOf (word.d);
    }
    break;
...
  case 'J':
    {
      int cindex = read_u2 (bytes, last);
      check_constant (pool, cindex, JV_CONSTANT_Long);
      _Jv_word2 word;
      memcpy (&word, &pool->data[cindex], 2 * sizeof (_Jv_word));
      result = Long::valueOf (word.l);
    }
    break;
...

while it seems like _Jv_word2 would always be twice the size of _Jv_word, the libjava/include/jvm.h header implies otherwise:
...
union _Jv_word
{
  jobject o; 
  jint i;           // Also stores smaller integral types.
  jfloat f;
  jint ia[1];           // Half of _Jv_word2.
  void* p;

#if SIZEOF_VOID_P == 8
  // We can safely put a long or a double in here without increasing
  // the size of _Jv_Word; we take advantage of this in the interpreter.
  jlong l;   
  jdouble d;
#endif

  jclass                     clazz;
  jstring                    string;
  struct _Jv_Field          *field;
  struct _Jv_Utf8Const      *utf8;
  struct _Jv_ResolvedMethod *rmethod;
};

union _Jv_word2
{
  jint ia[2];
  jlong l;
  jdouble d;
};
...

on a 32bit host, the _Jv_word2 probably is twice the size of _Jv_word (see the "jint ia[...]" lines).  but on 64bit hosts, both unions include a single jlong/jdouble entry which means they're probably both 8 bytes.  so the memcpy() in libjava overwrites 8 random bytes on the stack.