Put whatever unexpected work comes up into a general inbox somewhere, without preprocessing it (i.e. without figuring the next steps). Then periodically process

Put whatever unexpected work comes up into a general inbox somewhere, without preprocessing it (i.e. without figuring the next steps). Then periodically process what's in the inbox (figuring out the next steps and scheduling them), once a day, once a week, whatever. Also, buffer for unexpected stuff in your deadlines, in my work it's generally 40%.