Here is one of the better answers.
Failing to locate and eliminate Omar. Since Omar (and any of his associates) were the only links to Brother Mouzone’s assassination attempt, their failure should have been punished with their deaths. Failing to do this caused Bell problems later on leading to his death.
Not using Avon Barksdale’s imprisonment as the opportunity to seize control of the organization for himself. By isolating Barksdale in prison, Bell could have run the organization as he saw fit and actually turned the prospects of the organization around into a less violent, but more profitable enterprise.
Only killing D’Angelo Barksdale and not also Avon and Avon’s sister. A clean sweep of the Barksdale family would have allowed Bell to run the organization as he thought he needed to and removed an unnecessary level of burden to an already complicated situation.
Not using a knowledgeable intermediary to deal with Senator Clay Davis.He was clearly out of his league with Davis and had he used an attorney with the correct political connections, he could have likely gained all that he sought with fewer complications than he did.
Admitting that he had killed D’Angelo Barksdale. This very likely angered Avon Barksdale and led him to to betray Stringer more than the potential for his organization to cut off from a quality supply of product.
Stringer Bell had no mentor nor close adviser apart from Avon Barksdale. Had he had a "Butch" like Omar, a number of the obvious traps he was entering he would have easily avoided as they would have been pointed out to him.
Stringer Bell had few if, any men, that were strictly loyal to him. This lack of loyalty was exhibited in the fact that men loyal to him would have avenged his death regardless of input from Avon. He also seemed to have a very poor resource of internal and external spies to gather information for him.
Stringer Bell failed to properly use the police more effectively than he did. He was in possession of enough information to bring down his competitors, yet only used it against Avon and not the others. Any crime boss knows that cops are usually his second best tool against his enemies.
Bell failed to recognize that leaving the business (and thus Baltimore) was open to him. As the character was portrayed in the series, he clearly had no close family or friends in the city and could have easily departed a wealthy man to enjoy the fruits of his labours elsewhere. He chose to stay in a business that he eventually knew would either led to his imprisonment or his death.