The Treaty of Versailles was controversial for many reasons, both in Germany and in other countries such as the USA. Germany, in particular, may have had a substantial faction that would prefer to keep fighting (either because they believed victory was possible, to delay in hopes for a better deal, or in the "go down with the ship" kind of mentality, especially when faced with humiliating terms of the Treaty).
Others, perhaps, would hope that they could obtain better terms via America's help (contrast with Wilson's Fourteen Points) than by giving in to the Treaty, and that other powers would be more likely to negotiate than to resume hostilities, which (especially in war-weary democracies) nobody really wanted to do after suffering so many war losses, and after the public in those countries already accepted that "the war is over" and were mentally prepared to return to civilian life.
Additionally, events such as the Scuttling at Scapa Flow were ostensibly performed due to a "misunderstanding" about whether the Armistice was, in fact, in effect, and while it may have just been a convenient justification, it nevertheless pointed to the idea that a permanent peace treaty was not all certain before it was signed.
Was it ever a realistic possibility that Germany would refuse signing the Treaty, and continue fighting, possibly gambling for a better deal?
Indeed there was, after the treaty was presented to the German delegation, nearly all of the military generals wanted to fight and the chancellor at the time also supported the refusal of the treaty. However there wasn't enough support from the rest of the government so it was dissolved and the new government signed the treaty after the Allies gave them an ultimatum to respond in an hour or risk invasion.