Exact Valgrind version is current trunk r13884. The important lines from the stack trace:

==6232== Invalid read of size 1
==6232==    at 0x57B53B1: vfprintf (vfprintf.c:1630)
==6232==    by 0x57DD441: vsnprintf (vsnprintf.c:120)
...
==6232==  Address 0xe0cb310 expected vs actual:
==6232==  Expected: stack array "caseBuffer1" of size 1,000 in frame 3 back from here
==6232==  Actual:   stack array "caseBuffer2" of size 1,000 in frame 3 back from here
==6232==  Actual:   is 1,008 before Expected

The contents of the buffers are fine, I double checked. vfprintf implementation seems to apply some kind optimization which reads multiple bytes at once (probably 16 bytes). This is the assembly operation in question:

 repnz scas %es:(%rdi),%al



Reproducible: Always