Debug assertion fail

Well, first thing I believe is that you are using MFC.

Most of the MFC's code has ASSERT macros that show this dialog box, when something is not proper. For example, you write a function StrLen, which accepts a character-pointer:

int StrLen(const char* pString)
{
  // Iterate through string to find length
  // and return length
}

what is caller makes a mistake like:

char* p = NULL;
StrLen(p);

In this case, your function would cause access violation. Well, you can use defensive programming using ASSERT as follows:

int StrLen(const char* pString)
{
  ASSERT(pString != NULL); // Must not be null
  // Iterate through string to find length
  // and return length
}

In this case, if null (not empty string) is passed, your StrLen is defensing against null pointer, and this would cause "Debug Assertion Failed" dialog box.

An MFC method, like SetWindowText, would most probably defense itself for two concerns: Ensure the CWnd* (the this pointer is value, AND the window itself is value window/control. It is achieved through:

ASSERT(this != NULL);
ASSERT(::IsWindow(m_hWnd));

and if any of assertion fails, you get that dialog box. Just click Retry and you will see a call stack. Find the bug and correct it!