I have often seen RACI filled incorrectly and have blogged earlier that readers may want to refer. However, I would like to call out the following two issues further as they have relationship on other aspects of project management knowledge areas.
1. Mixed roles with "A" and "R": When a person or function is marked with both these roles, then this may introduce the risk of project schedule slip. If the individual responsible for doing the function fails to perform, then typically the accountable person will monitor the slip and ensure that the work is getting done. Alternatively, if the work is not completed satisfactorily, the accountable person shares the onus to check on the quality and the cost of poor delivery. Howevver, if the "R" person also is the "A" person, then the latter will not put any pressure on the former because they are both the same. This impacts risk, time, cost, amd quality. Similar challenges can be seen with "R" and "C" or "I" overlap.
2. Too may "A"s: If two people are accountable, then there are two types of problems. The first is the blindness game each "A" role plays thinking that the other role will keep an eye in ensuring the task is completed. When this taks fails to be done, the blindness game becomes the blame game reasoning with the "I thought you would have done it." This introduces project delays that may impact time and also introduces challenges with procurement. The second issue is the team gets conflicting directions from each "A" person leaving the team to get caught between power plays. The resulting team dynamics may lead to HR amd stakeholder challenges. Needless to stay that these may further impact other areas of project management.
There are several symptoms that a proper RACI may resolve for the project manager to proactively address. But unless a project manager is having a good understanding of RACI, the symptoms deteroriate leading to major problems requiring surgical intervention from executive management. The project manager can avoid these strategically by planning to succeed with end in mind.