I have seen situations where it did not matter and then some where it became a disaster. (some of this is in the context of in the lesbian community, often people will flock to one or two known lesbian therapists) if people talk and compare notes it can be a problem, if the therapist does different things with each person (as is often and even rightfully the case) it can cause problems. The problems I have seen are not so much in terms of the therapist - but in terms of what people do outside of therapy (discuss it with each other a lot and make judgements, get their feelings hurt, criticize how the other works with the therapist or relates to the therapist).
It has not usually happened with me because I don't usually talk to anyone about therapy nor do I see a therapist who is known in the community. I have seen friends have extreme jealousy and hurt feelings though.

The one I see did also see someone I knew and we all three knew about it. It only was a problem once when the therapist told me I "should" help the other person professionally on a matter we all knew about. I set that straight pretty firmly and it never happened again. I did not really worry the woman was talking about my particular issues with the other client though and she never discussed the other client's therapy issues with me.