As with the Boeing thread, I must apologize for being late to comment. But, interesting topic none the less.
Airbus' Biggest Mistakes
1) Underestimating the advantages of ETOPS. When Airbus launched the A330/340 Program, they stated it was their belief that a 4-engined aircraft is more economical to operate over distances of 5,000+ nautical miles than a twin. As the 767-200ER/-300ER had proven, and the 777 and its own A330-200 was about to prove, this simply was not true.
2) Limiting the range of the A300/310. With the introduction of the 767-300ER, Boeing had a winner on its hands. However, Airbus already had a decent competitor in its barn, it just lacked sufficient range to truly compete. The reasons for not expanding the role of this program is unclear to me, whether it was technical or philosophical. However, Airbus not only missed a chance to create a good 767 fighter, but also to refresh an aging product line, a decision they are now paying for.
3) Not designing growth into their aircraft. While on the techincal side, Airbus and Boeing build very similar aircraft. Philosophically, however, they have had one major difference. While Boeing has built their base aircraft with the ability to easily grow in the future (737-700 to -800/-900, 757-200 to 757-300, 767-200 to -300 to -400, 777-200 to -300), Airbus had maintained a design goal of optimizing the design for that specific model. The disadvantage of this policy can be seen in the A321 and the A340NG. The A321 suffers from performance shortfalls compared to the base A320, and specifically, to the nearest Boeing competitor at the time of its (A321) launch, the 757. Meanwhile, Airbus has spent considerably more time, money and effort developing the A340NG than Boeing has had to do with the 777LR, while the end result are very comparably aircraft. This philosophy has now changed for perhaps the one aircraft that won't end up needing the growth potential - the A380.
What are NOT mistakes
1) Not designing growth into their aircraft. Can something be and not be a mistake? Absolutely. The same philosophy that stuck Airbus with the A321 and cost them the A340NG, also allowed such aircraft as the A319 and A330-200, two aircraft that are highlights of their respective families. Especially the A330-200, an aircraft that is perhaps the best aircraft Airbus has ever built. They would not enjoy the performance advantages they have now if not for Airbus' policy of designing aircraft for a very specific size, which allows any shrink of that design to be a superb performer. But have they gone too far? I fear the same fate awaits the A318 as that of its trans-Atlantic rival, the 737-600 - too much for too little.
2) Scrapping plans for the A330-500X. IMO, this aircraft would have been a disaster. In much the same way as the A318, this aicraft would have been WAY overbuilt for its intended mission, carrying so much dead weight that it would have had trouble being even marginally economical to operate.
All gave some. Some gave all.