For memory allocation functions, Memcheck validates whether the arguments passed to them actually make sense. One example of this would be the passing of a negative size, or if it would far exceed a reasonable allocation size: for example, an allocation request for a petabyte of memory. Most likely, these values would be the result of a faulty calculation earlier in the code.

Memcheck would report this error like in this example from the Memcheck manual:

==32233== Argument 'size' of function malloc has a fishy (possibly negative) value: -3
==32233==    at 0x4C2CFA7: malloc (vg_replace_malloc.c:298)
==32233==    by 0x400555: foo (fishy.c:15)
==32233==    by 0x400583: main (fishy.c:23)
  

Here it was attempted to pass the value of -3 to malloc, which obviously doesn't make a lot of sense. Since this is obviously a nonsensical operation, it's indicative of a serious bug in the code.