The BrowserVerifyProperty will fail with a "BrowserEngine: 29 - Invalid locator or DOM element handle" if the property to be verified does not exist yet.  This can happen if the page is being updated by AJAX but the update has not occurred yet.  A solution for this is to use the BrowserWaitForProperty with a given timeout to ensure the property exists before trying to verify its contents like so:

transaction TMain
wnd1 : number;
BrowserStart(BROWSER_MODE_DEFAULT, 734, 367);
BrowserNavigate("", " (#1)");
wnd1 := BrowserGetActiveWindow("wnd1");
BrowserWaitForProperty("//H2[@hidefocus='0'][5]/A[@target='_self']", "textcontents", "Better software, faster", 6000);
BrowserVerifyProperty("//H2[@hidefocus='0'][5]/A[@target='_self']", "textcontents", "Better software, faster", VERIFY_EQUAL, SEVERITY_ERROR);
end TMain;

Further information on the BrowserWaitForProperty, including required parameters is available in the SilkPerformer help files.