We recently found and fixed a bug which may silently cause spikes to be lost or extra spikes to be generated in certain circumstances. At the moment, the only known example of a problem is when using SpikeGeneratorGroup in a simulation with multiple clocks as in this message on the support group, but the problem may also appear in rare situations with other simulations with multiple clocks. We advise users to upgrade to Brian 1.1.4 when it is released next week.