Sometimes AWS spins up bad instances. Usually sheriffs notifies ciduty about these but if you see one escalate to ciduty in #ci. A job may appear as failed if the instance it was running on disappears. Spot instances can disappear when they are outbid.
Bad Instances
To understand if a job failure is caused by a spot instance or not it's best to first understand the various ways a task can be resolved. See this page for more information.
When AWS spins up a bad instances (usually identified by the fact that it fails every job), find it in the worker explorer of AWS Provisioner and quarantine it. Its inactivity will cause the worker to be terminated and AWS will spin up a new one. You can do this even while a task is running due to the built in mechanism for retrying jobs. To further understand the interaction between the queue and a worker, check out the official docs.