Gmail was down a while. Google describes how it happened.
In essence, a mechanism designed to throttle load on heavily used parts of the infrastructure reduced total capacity. If demand is then not reduced this leads to congestion, similar to what happens in a traffic jam.
One way for gmail to reduce demand is to signal to the webbrowser to decrease the frequency with which it polls the gmail servers for new mail (I do not know if they already do this).
On a side note: can anyone think of a way to beta test systems of this size?