Thamsanqa Jantjie, the sign language interpreter at Nelson Mandela's memorial service accused of signing nonsense, who has since said he was having a schizophrenic episode, may have inadvertently done deaf people around the world a huge service in bringing public attention to the issue of poor quality sign language.

While inaccurate interpreters on a world stage are rare, bad interpretation is surprisingly common and something that deaf people who use interpreters face on a regular basis. Across public services and the courts unqualified people are asked to translate, even in situations where clear communication can make the difference between life or death. A friend's father who had terminal cancer had to make do with a hospital porter who had basic sign language to help him communicate with the doctor. Frequently medical or other professionals think it is OK to ask young children to interpret for their parents.

Jantjie claims that he knew that he was having a schizophrenic attack but had to keep going. Any professional interpreter would understand that if they found themselves in a similar situation, they would have to stop. Had another interpreter been booked - which should have been the case given the high profile nature of the event and how long it was - there would have been back up.

Demand for signers currently far outstrips supply, and this has created a market for cowboys – people who may be able to sign but who have no or little training in interpreting. The qualified people come with a gaggle of incomprehensible qualifications – they might be NRCPDs, they could be MRSLIs, or simply RSIs. Only a handful of people know what these mean, and I am certainly not one of them. On top of this, there is a general lack of awareness as to what a sign language interpreter is: someone who is fluent in the language and trained to translate, rather than just someone who knows a few signs. So it's easy to see why people with the best intent may book someone who is unqualified.

Once at a job interview, I had an interpreter who was struggling painfully to speak for me and even fingerspelling words slowly didn't help her. At various points the interviewers had to step in to tell her what I was saying. Perhaps the bond I created with the interviewers as we exchanged glances and chuckled at the situation helped me to get another interview and, eventually, the job. But they could quite easily have got cold feet about employing a deaf person after that kind of experience – and I wouldn't really have blamed them.

I have a list of excellent interpreters I use regularly at work but if something comes up at the last minute and none of my contacts are free, I have to branch out into the wild west. Outside of the safety of my trusty list, I have no idea if the person I've booked will be able to do the job. I had someone interpret for me a few months ago who signed as though she had weights strapped around her arms and would condense long comments I said merely into "yes". The worst thing about these situations is the paranoia that I am coming across as incompetent and inarticulate because others can't tell the interpreter isn't up to scratch. After all, millions of people watched the Mandela memorial service, not realising something was amiss with the signer.

As people across the world discuss this tale, they are learning that getting up and moving your hands does not make you a sign language interpreter.