Super Smash Bros. Brawl utilizes a double-layer disc which has a large memory capacity. A very small percentage of Wii consoles may have trouble consistently reading data off this large capacity disc if there is some contamination on the lens of the disc drive.
In case you have this issue you should contact Nintendo consumer Support. Nintendo has specialized cleaning equipment that can resolve this problem. You will have to send in your Wii to Nintendo to have the disc drive lens cleaned. There is no cost for the repair or shipping.
More details on the Nintendo Europe site.