Only once at my previous job. I was in a meeting with a potential customer! However the circumstances should explain.
We had a project that had to be ready on a Friday at 6 PM. Everything was on track. Came in Thursday morning and over the previous evening, someone had tried to run the machine and crashed it! We started working on it first thing Thursday morning and worked straight through until 6 PM on Friday. I was in the office and out on the floor working on that machine for 36 hours.
As I was headed back to my office to get my keys and go home, my boss caught me and wanted to introduce me to a potential customer and asked me to explain the capabilities of our electrical engineering department. I was fine while I was talking but the customer started explaining what they needed and I fell asleep. I explained what we had just finished doing and he was very nice about it. I think he was actually upset with my boss for pulling me into the meeting when I hadn't slept for 36 hours and needed to go home.
As I recall, we did get some business from this customer so it must not have influenced him too much.