if there is a bug in swarming task template or swarming is down, swarming task creation will fail and an allocated a build number will be forever lost. If build creation rate is high, this will cause large gaps in build numbers.

It is somewhat problematic because users like their short numbers and a large gap can increase number of digits e.g. from 2 to 4.

I mark this as p2 because this is a result of a larger issue (swarming is down, swarming task template is broken) and we should avoid THAT in the first place.