In the February issue of Software Test & Performance read Karen Johnson's article about multi-user testing (pp. 20-23). Karen writes about a very rare subject – functional multi-user testing. Should admit that I started to read with a thought "one more article about performance testing" – but soon realized that it is about quite different subject. And yes, indeed, without functional multi-user testing, most of errors mentioned in article will slip through (won't re-tell the article – it is available on-line, you can read it yourself). Some may be found during performance testing (probably the most severe like deadlocks – if they are in the typical scenarios you included in performance testing) – and you will need to trace them down to the source, and probably it will be much later down the cycle.