I've made so many financial mistakes it would be hard to figure out which was the biggest, but the one that's currently occupying my mind is one I'm currently faced with: as a first-time homebuyer last year I took advantage of the Roth IRA first-time homebuyer provision, which allows you to withdraw up to $10K of your earnings toward your first home. It helped, as we didn't have quite enough for our targeted 20% downpayment. I had two IRAs that I couldn't contribute to anymore (because I now live in Canada), and together their total value was about $11K so I cashed them in.
However, while I correctly determined that this wouldn't have any effect on my US tax liability, I didn't think about the fact that it would count as income on my Canadian taxes. And since I'm in the highest marginal tax bracket in Canada, it turns out this will cause me to owe more than $5,000 in Canadian income taxes. My accountant gave me the bad news yesterday.
If on the other hand I had used the similar first-time homebuyer provision in my Canadian RRSP account, it wouldn't have counted as income and I wouldn't have had to pay any tax on it. Live and learn...the hard way.