One of our Jira-savvy users attempted some fancy regex in JQL on the standard issue search screen. To be exact:
issueFunction in issueFieldMatch("", "Category", "^Dev.*")
This query brought our production Jira instance down while under load and the attached screen shot shows the graph from our stage instance where we tested under zero load and still got 50-60kms spikes in web response times, though no outage.
We believe this is a significant bug in that it allows average users to cause critical impact to the JVM via the UI.