The HTML generated for C code has the occasional extra </span> tag.  Some HTML parsers (including the one in swish-e) declare errors on the extra tags.

The problem is in endCodeLine() in code.cpp (although the same problem exists in code.l, fortrancode.cpp, fortrancode.l, latexgen.cpp, etc.).  This is the original function.  It writes a closing tag, but doesn't set g_currentFontClass to 0.  That lets a later point in the code send an extra closing tag.

#line 8852
static void endCodeLine()
{
  if (g_currentFontClass) { g_code->endFontClass(); }
  g_code->endCodeLine();
}

The fix is obvious:

static void endCodeLine()
{
  if (g_currentFontClass)
  {
    g_code->endFontClass();
    g_currentFontClass=0;
  }
  g_code->endCodeLine();
}