In some scenarios DVCS connector is reporting a Rate limit exceeded error in the logs (429) which will look something like:

2015-06-18 07:13:47,461 DVCSConnector.MessageExecutor:thread-1 ERROR      [dvcs.service.message.MessageConsumer] Synchronization failed: Error response code during the request : 429
com.atlassian.jira.plugins.dvcs.spi.bitbucket.clientlibrary.request.BitbucketRequestException$Other: Error response code during the request : 429

When the rate limit for Bitbucket is exceeded - https://confluence.atlassian.com/display/BITBUCKET/Rate+limits

The next sync will carry on from where the previous failed so it will eventually complete but any syncs triggered whilst the rate limit is breached will show this error.