I want to know why errors appear under the wrong issue - either different errors share one issue, or related errors are split across many issues.

Sentry groups events that share the same fingerprint into a single issue. For errors, Sentry's default grouping algorithm evaluates the fingerprint, stack trace, exception, and then message to determine which events belong together.

To understand why a particular event was grouped into an issue, open Issue Details and expand Event Grouping Information at the bottom of the page.

If you already have separate issues that should be treated as one, you can merge them. Merging is useful for cleaning up existing issues, while grouping rules determine how new events are grouped.

To change how new events are grouped, you can use Fingerprint rules, Stack Trace Rules, or SDK Fingerprinting. These rules don't change how issues that have already been created are grouped.

Transaction and performance issue grouping can't be customized.

For a broader overview, see Issue Grouping. If you're using both project-level fingerprint rules and SDK fingerprinting, see How can I use server-side fingerprint rules together with SDK fingerprinting?.