Depends on what you mean by "mistake." I am disappointed by the RROD issue, but not surprised... new technology often has it's drawbacks and glitches. I am pissed by their denial of the problem for the last year and a half, but that's standard marketing and I think it was much less of a "mistake" than the opposite would have been. If they had come out and said "Yeah, nearly 1/3 of all 360's out there will die" no one would buy one and they'd have no market share for Sony to try and take back. So, business-wise, neither of those are really "mistakes", in my book.
What I consider to be their biggest mistake isn't listed on the poll, and that is the fact that not every 360 has a HDD, so no developer can count on it to be there. Sure, some developers may take advantage of it if it's present, but realistically most developers will write the code assuming you DON'T have a HDD, thereby making the HDD simply a glorified storage device and bypassing all the caching and other improvements that the HDD gave in the original XBox. This holds especially true given the time crunch the developers already go through... not many will have the luxury of taking the time to write extra code to optimize the use of a HDD.
Just my two cents.