From a personal standpoint, guilt. I've asked so many times for forgiveness from people that I've hurt, and usually are received. Though people can give you a straight yes or no. God's response is always your interpretation of his Word, and how he's involved in your life.
Of the common, forgivable sins, I have too many for me to name, let alone account for with asking forgiveness specifically. But the ones that haunt me the most, in regards to the topic is blasphemy. Something of a specialty of mine, in my youth, and still today. I can't say as though I've fully figured out this one thing when it comes to Christs teachings, so until I do it will bother me, and make my salvation always just beyond my fingertips.