The valgrind job captured "invalid read of size" errors in a test/format run. The memory block was allocated in a page eviction logic, with a large block size freed later during a reconciliation.

Address 0x73e4fa8 is 40 bytes inside a block of size 56 free'd 

The link to the valgrind result:

http://build.wiredtiger.com:8080/job/wiredtiger-valgrind/3377/valgrindResult/pid=59624/