The errors that PurifyPlus discovers include array bounds reads and writes, trying to access unallocated memory, freeing unallocated memory (usually due to freeing the same memory for the second time), as well as memory leaks (allocated memory with no pointer reference). Most of these errors are not fatal (at least not at the site of the error), and often when just running the program there is no way to detect them, except by observing that "something" is wrong due to incorrect program behavior. Hence PurifyPlus helps by detecting these errors and telling the programmer exactly where they occur. Because PurifyPlus works by instrumenting all the object code, it detects errors that occur inside of third-party or operating system libraries. These errors are often caused by the programmer passing incorrect arguments to the library calls, or by misunderstandings about the protocols for freeing data structures used by the libraries. These are often the most difficult errors to find and fix.