Overrun and underrun errors occur when your program writes outside of the bounds of the allocated block. They're one of the most difficult type of heap corruption to track down, and usually the most fatal to program execution.
Overrun errors occur when the program writes past the end of the allocated block. Frequently this causes corruption in the next contiguous block in the heap, whether or not it's allocated. When this occurs, the behavior that's observed varies depending on whether that block is allocated or free, and whether it's associated with a part of the program related to the source of the error. When a neighboring block that's allocated becomes corrupted, the corruption is usually apparent when that block is released elsewhere in the program. When an unallocated block becomes corrupted, a fatal error will usually result during a subsequent allocation request. Although this may well be the next allocation request, it actually depends on a complex set of conditions that could result in a fault at a much later point in time, in a completely unrelated section of the program, especially when small blocks of memory are involved.
Underrun errors occur when the program writes before the start of the allocated block. Often they corrupt the header of the block itself, and sometimes, the preceding block in memory. Underrun errors usually result in a fault that occurs when the program attempts to release a corrupted block.