Error reference

Reference guides to common production errors. Each entry links to an in-depth guide explaining root causes, code examples, and the typical fix. When an error like these goes uncaught on your server, the bugstack SDK captures it and starts a fix.

JavaScript / TypeScript

• TypeError: Cannot read properties of undefined — null/undefined property access in Next.js
• Unhandled Promise Rejection — missing await or catch in Express async handlers
• TypeError with React useState — incorrect state initialization or stale closures
• Hydration Mismatch — server/client HTML divergence in Next.js
• CORS Error — missing or misconfigured CORS headers in Express
• Memory Leak — unbounded listeners, caches, or closures in Node.js
• Module Not Found — broken imports or missing dependencies in Next.js
• ReferenceError — accessing undeclared variables in Next.js
• Request Timeout — long-running handlers without timeout guards in Express
• See all JavaScript error guides →

Python

• AttributeError: NoneType — accessing attributes on None in Django views or models
• ImportError — circular imports or missing modules in Flask
• TypeError: FastAPI Validation — Pydantic model mismatches in FastAPI endpoints
• Database Connection Error — connection pool exhaustion or misconfigured settings in Django
• Migration Error — conflicting or missing migrations in Django
• CSRF Verification Failed — missing or invalid CSRF tokens in Django forms
• TemplateDoesNotExist — missing or misconfigured template paths in Django
• DoesNotExist — querying for non-existent records in Django ORM
• Blueprint Error — incorrect blueprint registration in Flask
• See all Python error guides →

Ruby

• NoMethodError: nil:NilClass — calling methods on nil in Rails controllers or views
• ActiveRecord::RecordNotFound — querying for records that don’t exist
• RoutingError — unmatched routes or missing controller actions in Rails
• Migration Pending — unapplied database migrations blocking startup
• ArgumentError: wrong number of arguments — method arity mismatches in Rails
• Asset Pipeline Error — missing precompiled assets in production
• N+1 Query — excessive database queries from missing eager loading
• CSRF Token Error — invalid authenticity token in Rails forms
• See all Ruby error guides →

Go

• Nil Pointer Dereference — accessing fields on nil structs in Gin handlers
• Index Out of Range — slice or array bounds violations
• Panic Recovery — unrecovered panics crashing goroutines
• Channel Deadlock — blocked channel operations causing goroutine deadlocks
• Context Canceled — premature context cancellation in HTTP handlers
• JSON Unmarshal Error — type mismatches when decoding JSON payloads
• SQL Connection Pool Exhaustion — leaked database connections in Go services
• HTTP Client Timeout — missing or insufficient timeout configuration
• Race Condition — concurrent map access without synchronization
• See all Go error guides →

Don’t see your error?

The SDK does not depend on this list. It captures any uncaught server error, catalogued here or not. Specialist agents work out the cause and write a small change, it arrives as a pull request that runs through your CI, and what happens next depends on the autonomy level you set.

• Get started with the quickstart guide →