It will darken the brew more than designed.
If you mixed it in well, no harm. If you let it sit on the bottom while the water came to a boil, and it burnt, then you would have ruined your batch. You would know this by seeing LME stuck to the bottom of your pot after emptying it.
OK so I finally made a newbie mistake after two great batches to start off my brewing hobby.
When brewing my St Patrick's Irish Stout Deluxe edition, I misread the instructions that came with the LBK and assumed the LME pack and a booster were the same thing (never having used either before). So I put the LME in the pot while the water was still cool, then brought to a boil, removed from heat, and added the extract. Realized too late that the LME and extract were both supposed to go in after the water was removed from the heat. Will this screw up my beer or is there still hope this one will come out OK? After a day, the fermentation looks like it is doing its thing. Fingers crossed I didn't botch this from the start.