A program that works in a particular memory allocation strategy may abort when the allocation strategy is changed in a minor way.
A good example of this is a memory overrun condition (for more information see "Overrun and underrun errors," below) where the allocator is permitted to return blocks that are larger than requested in order to satisfy allocation requests. Under this circumstance, the program may behave normally in the presence of overrun conditions. But a simple change, such as changing the size of the block requested, may result in the allocation of a block of the exact size requested, resulting in a fatal error for the offending program.
Fatal errors may also occur if the allocator is configured slightly differently, or if the allocator policy is changed in a subsequent release of the runtime library. This makes it all the more important to detect errors early in the life cycle of an application, even if it doesn't exhibit fatal errors in the testing phase.