One of my colleagues has a sign in her office that reads: "Life isn't perfect, It's messy!" A client initially made it as an affirmation and reminder to herself, but left it with the clinician after completing treatment. I think its presence in the office is a way of passing that affirmation and reminder on to future clients - a piece of hard-won wisdom.
As important as it is for clients to come to terms with the impossibility of attaining perfection, it's also important for clinicians to realize. We try to be the epitome of professionals. To always say and do the "right thing" and the "right time." To never let our own feelings, assumptions, or worldview cloud our judgment. To avoid revealing our own imperfections and vulnerability to clients (or colleagues, or supervisors).
I see this particularly in students and new staff who worry about making a mistake, then fret and feel bad when the inevitable mistake occurs. I also see it in myself when I have a day when I'm not at my best - when I forget something or show frustration or inadvertently upset someone....
Thankfully, doing good therapy does not require us to be perfect, only that we be "good enough." Just as "good enough mothering" (a la Winnicott) suggests that imperfect attunement from a mother allows for (indeed facilitates) maturation in a child, so does imperfect counseling create opportunities for clients (and clinicians) to grow and work through old issues in new ways. In particular, our ability to repair and address our imperfections seems to be a rich opportunity to deepen the therapeutic relationship, and address the client's deeper interpersonal issues that may be less accessible at other times.
Yes, neither life, nor any of us humans, is perfect. We cannot expect perfection of ourselves, or others, and our clients do not (or should not) expect it from us. Life, and therapy, are messy. But that messiness becomes the foundation for creativity and growth, and that's what makes life - and therapy - worth it.