I wanted to say the A7V Sturmpanzerkampfwagen! (Storm Armoured Combat Vehicle) It was such a godawful design that the Germans had to capture British Tanks, calling them Beutepanzers, and use them!
Anyway, since Reggie used the A7V, I must make do with the Schneider CA1. The French attempt at a WWI tank:
Now, I should explain that at the outbreak of WWI there were two types of track set. The Holt trackset, which was originally designed by a British, Lincolnshire Firm (Rustons) and then exported to America, where it was modified to make tracks suitable for praire sowing vehicles. These Holt tracks were then modified further to become the second type available, Bullock tracks, also for crossing prairies.
Now, it became obvious early into the war to the boffins behind "His Majesty's Landship" that neither track set would be suitable, as they were designed for prairies and the tracks simply fell off when they came into contact with rough ground. Thus they designed the rhomboidal track system we know so well today, which was due to its unsprung, iron plate design, was capable of going pretty much anywhere.
The French, however, were not privy to any of this British research, and after the British Mk1 came out, wanted in on the action. Now, British Mk1 was an excellent design, poorly constructed. It was so secret that the workers making it were called up, as they were not allowed to say what they were working on. As such, British tanks, whilst excellent designs, were built by unskilled, barely trained labour. That is why they were so unreliable.
But, and it is a big but, the French had no equivalent to Messrs. Wilson and Tritton (The British tank's designers) in terms of talent. Also, what with the British tank being Churchill's pet project, it received ample funding. The Schneider, on the other hand, did not. It was apathetically slapped together by giving an armoured truck more armour, and then bolting on a track system. A Holt track system, mind, as the French had neither the time nor the resources to agonisingly develop any better. As if that was not bad enough, the Tank was fitted with miserably inadequate weaponry, only two hotchkiss machineguns and a single 75mm cannon, far behind it's contemporaries. And then to add insult to injury, the hull-like protuberance at the front made the tank ditch itself whenever going up even a moderate incline
Suffice it to say that this made the Schneider slow, unreliable, poor on rough terrain and badly armed. French manufacturing capacity (and resource allocation to armour) lagged behind the British, meaning that, while Britain was churning out hundreds of Tanks (Medium and Heavy), France could barely squeeze out 130, most of which were unsurprisingly lost at their very first engagement, at Berry-au-Bac.
Incredibly, they were in operational use up until the Spanish Civil War, in which the Republicans fielded 4 (rather ineffectually).
Of course, the Schneider was not the only tank built by the French in WWI. The little Renault FT-17 was so good that the USA built it under licence, too. But this, appalling effort will remain a testament to how not to design a tank: with apathy.