I'll repeat what others have said again...in a professional setting, you have to document, document, document. In this instance you need to institute tools to help this employee keep track of assignments that have critical deadlines.

Don't just spring this on her. Have a meeting where you discuss some of the issues you have. Utilize some of the examples that have occurred. Let her know you are aware of the issue and still sympathetic to her as a worker.

Let her know that this is something you're instituting not for punitive reasons. You're protecting the goals of the office and the mission critical deadlines. You're also instituting this to help prevent future problems between employees like the ones that have occurred recently.

Stress this is something you're doing to prevent problems and document in the future so you can prevent disagreements.

You also owe it to your employees to discuss with them the new process and the reasons for it from their perspective. Don't take sides. Focus on acknowledging the conflict and focus this process as a way to document the issue moving forward.

From here on out, the he said/she said unspoken problems will be on paper and something you can use as a way to start a conversation via progressive discipline if necessary, but initially as coaching and mentoring sessions.

It takes time, it creates a process that adds friction in perms of potentially slowing down your office. But this needs to be a springboard for you to find other areas in your office and your business processes where a documentation system could replace word of mouth and just unspoken processes so that future issues like this one are avoided and you have the tools to nip problems like this in the bud using performance review sessions, coaching and mentoring before you ever have a public conflict between workers.

Conflict is a quick way to create cattiness and bad office morale and a poor work environment. Bad work culture sucks.