The build key uses the build number, so if things go wrong in a build environment that aren't related to the code itself, the overall commit is marked as failed, even if subsequent builds pass.

An option such as "Only show latest build status for each commit" that if toggled excludes the build number from the build key would allow commits to be rebuilt upon error.

Bitbucket has done a write up on this problem - Commit build status is always red even if other builds succeed

It looks like it may be simply updating BitbucketBuildStatusNotifier.java:299 to exclude the number if the configuration option is on.