How to debug Heap errors

[sedi]

Hi, wenn deleting an object (derived of a QGraphicsObject) I receive an error message:

Heap block at 0D6EC658 modified at 0D6EC700 past requested size of a0

The stacktrace shows nothing helpful (at least yet for me):

0 ntdll!TpWaitForAlpcCompletion C:\Windows\system32\ntdll.dll 0 0x77bf04e5
1 ?? 0 0x0028be90
2 ntdll!RtlLargeIntegerDivide C:\Windows\system32\ntdll.dll 0 0x77bd5732
3 ?? 0 0x0d5b82a0
4 ntdll!RtlCopyExtendedContext C:\Windows\system32\ntdll.dll 0 0x77bb29fa
5 ?? 0 0x00000000

(full Stacktrace attached)
The problem occurs when I allocate memory for a class member (QGraphicsRectItem*): whenever I initialize it with "new" the object will throw the above error on deletion.

This seems "funny" because I do that all the time in the base class of the same object (which base class is in turn the afore-mentioned QGraphicsObject): QGraphicsObject-->ButtonObject-->AppButton

As said: The ButtonObject has loads of QGraphicItems* of any kind and allocates them quite happily.

How can I trace the problem? It's about the first time I encounter this kind of error, therefore please don't assume common knowledge.