This happens when the page itself defines a global variable called console, for example. If the page is browser-sniffing to decide whether to define it, the behavior could differ in different browsers.
In the case of Firefox it also happens when Firebug is installed and its console is enabled, since that overrides the default window.console.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
