This phase I trial studies the side effects and best dose of veliparib when given together with irinotecan hydrochloride in treating patients with cancer that has spread to other parts of the body or that cannot be removed by surgery. Irinotecan hydrochloride can kill cancer cells by damaging the deoxyribonucleic acid (DNA) that is needed for cancer cell survival and growth. Veliparib may block proteins that repair the damaged DNA and may help irinotecan hydrochloride to kill more tumor cells. Giving irinotecan hydrochloride together with veliparib may kill more cancer cells.