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Production error deep-dives — root cause, reproduction, and the fix that holds.

Comparison Method Violates Its General Contract in Java

IllegalArgumentException: Comparison method violates its general contract! comes from TimSort, not your code. Here is why it fires, and how to fix the comparator.

Java InaccessibleObjectException: Causes and --add-opens Fix

InaccessibleObjectException: module java.base does not "opens java.util" to unnamed module. Why strong encapsulation breaks reflection and how to fix it properly.

Java IllegalStateException: Duplicate Key in Collectors.toMap

Collectors.toMap throws IllegalStateException: Duplicate key when two elements map to the same key. Exact causes, JDK 8-25 message differences, and the fixes.

Java UnsupportedClassVersionError: Class File Version F

java.lang.UnsupportedClassVersionError means your bytecode is newer than your JVM. Decode the class file version numbers and fix the mismatch for good.

Java OutOfMemoryError: Java Heap Space - Causes and Fixes

java.lang.OutOfMemoryError: Java heap space and GC overhead limit exceeded, explained: reproduce both, read the GC log, find the leak, and fix it properly.

Flink Heartbeat of TaskManager Timed Out: Causes and Fixes

Fix the Flink 'Heartbeat of TaskManager with id ... timed out' error. Real causes — GC pauses, OOMKilled pods, network saturation — and the exact config fixes.

Kafka TransactionalIdAuthorizationException: Fix the ACL

TransactionalIdAuthorizationException: Transactional Id authorization failed. Why your EOS/Spring transactional producer dies on a secured cluster and the exact ACLs that fix it.

Kafka CorruptRecordException: Record Is Corrupt — Fix It

Kafka CorruptRecordException: Record is corrupt (stored crc != computed crc). Why the CRC check fails, how to recover the partition, and how to stop it recurring.

Kafka's UNKNOWN_PRODUCER_ID and OutOfOrderSequenceException: Why Your Idempotent Producer Fails After a Restart

A deep dive into why idempotent Kafka producers throw UNKNOWN_PRODUCER_ID and OutOfOrderSequenceException in production, how broker-side producer state expiry causes it, and how to fix it with KIP-360 semantics and proper client configuration.

HikariPool-1 - Connection is not available, request timed out after 30000ms: Diagnosing and Fixing Connection Pool Exhaustion in Spring Boot

A production-grade breakdown of the HikariCP Connection is not available timeout in Spring Boot — why it happens under load, how to reproduce it locally, and how to fix and prevent pool exhaustion for good.