The International Space Station typically has one or more (manned) spacecraft docked. This is obviously to ferry crew to and from the station for each of the Expeditions. The Soyuz currently fulfils this role, but during its operation the Space Shuttle was also used.
In the event of an emergency the docked craft can be used to evacuate the ISS and return the crew safely to Earth.
At any point has there ever not been the capacity for a full evacuation?
Common sense says no - every crew member went up in a seat therefore there should always be one available for them to evacuate. Also crew safety is clearly the primary concern.
However, not all launches are full, not all landings are full and some astronauts stay for multiple Expeditions, meaning the number of crew doesn't always match the number of available seats. This assumes that a full evacuation means every crew member has a seat.
NASA had a plan for a permanent escape vehicle, but this was cancelled.
Edit: I'd like to be certain that there were no situations (for example during relocation of a Soyuz craft - though this case has been addressed). We can also ignore EVAs where technically prompt evacuation would have been difficult. Granted, this may be difficult to prove definitively.
Edit 2: Although the conclusions seem pretty clear, I'm still not entirely convinced that there was never a short period without full evacuation capacity. There is some good discussion on NASASpaceflight on the rationale behind always ensuring evacuation. This paper also details the studies into a dedicated ACRV.
In the absence of an official word from one of the organisations involved, or unless someone can find a cross-referenced chronology of all astronaut activity, I'll accept the top answer as it's certain to be correct!