lets assume some one screws up big.
That means, of course, that we get a nicely tangled-up mess. Just as in OTL, from an early point, US public opinion will see a majority leaning anti-Germany (a position that became consolidated when the first news of atrocities in Belgium made the news). The peace-time USA will, is in OTL, see American banks extending huge loans to Britain and France. So up until somebody messes up in a huge way, the expectation will be continuing friendly-to-the-Entente neutality, or active entry on the Entente side.
But then, the board gets flipped. The USA is now in an unexpected position. Wilson will, as in OTL, have made only the barest minimum of preparations for a land war. The army will still be on its modest peace-time footing. Regarding public opinion: anything dramatic enough to get Congress to declare war on the Entente powers will no doubt also be sufficient to get the populace clamouring for this. (In fact, we may expect it to be the other way around: since entry on the side of the Central Powers is a wild swerve at this stage, it seems likely that public opinion essentially forces the decision on Congress -- on penalty of a major electoral shift come 1918, should Congress refuse.)
This means that the POD must be dramatic indeed. Supposing the Brits do something Lusitania-esque, even, the the expected reaction is that the USA demands apologies, assurances that it'll never happen again, and of course compensation. We may safely assume Britain would grant all this at once. Even if it messes with their whole Atlantic strategy and the american demands basically give the germans free reign... that's still better than having the USA enter on the German side. So that's saying something. There must be some War-of-1812 type of animosity building up, here.
But let's assume something really bad happens, as a direct result of British policy, and that a combination of wrong-guys-in-the-wrong-jobs and miscommunications, against a back-drop of a Britain that feels it's backed into a corner, leads to a conflict neither side wants, but both sides think is unavoidable. Stranger things have happened.
What would the outcome be? The USA would, for one, naturally cease all its support for the Entente. Instead, all such efforts would now go to backing the Central Powers. Quite actively so, because with britain as an enemy, a British/Entente victory now means that the USA never gets any of its loans back. Conversely, a Central Powers victory means the USA can demand all it is owed (and more) as part of the peace settlement after the war. (Germany, at the moment thanking God most profusely, will be all too happy to back any claim the Americans wish to put forward come that day.)
The Entente is now in major trouble. The Royal Navy can still dominate the Atlantic, and can presumably intercept any material support the Americans might send to the Central Powers. But that doesn't matter. The fact is, having expected American loans to continue, the Entente is now in a lurch. They're already leveraged to the hilt. Finding new benefactors will be a non-starter at this point. Especially so because it now seems near-certain that they'll lose.
After all, Britain can now choose between sacrificing Canada, or trying to defend it... in which case they'll have correspondingly less means and men to dedicate to other theatres. The war has just become a no-win scenario for the Entente. Theoretically, their best option is to immediately sue for some semblance of "peace with honour", if they can get it. Germany might be reasonable (it being 1916), and then the war ends before the USA even has to do anything. The USA will demand reparations for whatever slights it has suffered, and the Entente will have to pay its debts, but otherwise, the war in the West ends with a Central Powers victory. In the East, Russia may become the victim of German ambitions -- which can now be realised without sending over mr. Lenin. The Germans will have enough men freed up to overwhelm the Russians. Of course, this also depends on German demands and the Russian response. If Germany isn't too greedy, Russia (now alone) may just cut its losses. If German demands become outrageous, Russia probably will fight on, hoping the Germans will tire of war come winter.
There is the alternative scenario, of course. Germany might be so emboldened by US entry that the Kaiser feels ge can go wild with his demands all-round. To such a degree that the Entente fights on, despite the dim prospects. The war rages on. The financial realities will catch up to the Entente, and they will eventually collapse. German demands at the war's end will be as vindictive as can be. Meanwhile, the American part in all this will boil down to: all those guys who went to Europe in OTL are marching North instead. At the end of the war, the USA will simply be the new owners of Canada. ("You may now cede the parts we haven't occupied yet, and if you don't, we'll add the expected costs of our trouble to your war reparations.")
The outcome of this second scenario are hard to estimate. Veering into speculation, I can imagine the collapse of the British Empire. France would be a mess, too. Italy would be humiliated. On the winning side: the USA, its work cut out for it, having to digest Canada. (Would probably demand Bermuda and the British Caribbean, too -- for strategic reasons.) Germany, Austria-Hungary and the Ottomans would all be bloated by their post-war annexations. Lots of unhappy new subjects all over the place. Germany would probably demand a lot of colonies, too. What it can't annex, it might just seek to cut off from Britain and France altogether. Suddenly-independent India? That's an option. Anyway, there's a good chance that not only Russia goes red, but France, Britain and Italy as well. (United against Mitteleuropa, common cause makes the most sense. So even if some kind of fascism wins out, it'll lean left to close ranks with the Soviets.) If Britain really goes red, places like Australia will almost certainly go independent.
All in all, it would be a huge mess, and a second war would be inevitable.