VBox: VMs randomly fail with assertion: Expression: cbPreRead == 0

[Yuri]

Here is the message:
00:10:44.838619 !!Assertion Failed!!
00:10:44.838619 Expression: cbPreRead == 0
00:10:44.838621 Location  : 
/usr/ports/emulators/virtualbox-ose/work/VirtualBox-4.2.16/src/VBox/Storage/VD.cpp(2272) 
int vdWriteHelperOptimized(VBOXHDD*, VDIMAGE*, VDIMAGE*, uint64_t, 
size_t, size_t, size_t, size_t, const void*, void*, unsigned int)

Gues OS: FreeBSD 9.1 (both i386 and amd64)

virtualbox-ose-4.2.16_2

Yuri