It's pretty easy to burn toffee. The butter in the mix is prone to burning at a lower temperature than 300º. Turn the heat lower and when the mixture gets close to the target temp, start stirring continuously.

It took me three times to get my toffee right this year. I wasn't stirring it enough, and as others have noted, it scorched in a few spots. Also, check the accuracy of your thermometer by testing it in boiling water. Good luck!

Do you add a bit of Karo syrup to your sugar? Careful stirring is important, especially around the edges of the pot. I make my toffee with the burner just under medium heat.

Hmm...did you use a less refined sugar, like turbinado? Sometimes the toffee can burn due to the larger quantity of impurities. Also, if you're at a higher altitude, most recipes involving boiling sugar (for caramel, toffee) recommend lowering the temp by 10 degrees or so. If chocolate is added to the mix, that could also be a culprit. These are just a few ideas -- I'm sure there are other factors.

Sometimes a few stray sugar crystals will stick to the bottom of the pot and if not caught and stirred in they will create a hot spot and cause the candy to scorch. Either that or the calibration on your thermometer is off and the candy got too hot - it only takes a few extra degrees to go from delicious candy to a burnt mess.