After we raised a Lambda timeout to handle large PDFs, customers started getting the same invoice email twice. The SQS queue's visibility timeout was still 60 seconds, so slow messages reappeared and a second invocation picked them up mid-processing.
A refactor changed one output prefix in an image processing Lambda, and the function started triggering itself. Invocations went from 40,000 a day to 9 million in six hours. We stopped it with reserved concurrency, then separated input and output and added loop guards.
Our API Lambda logs carried a correlation_id from API Gateway, but the SQS consumer logs had none, so we could not follow a request across the queue. Powertools only reads correlation IDs from the event it is given. How we passed the ID as a message attribute and injected it per record in the consumer.
Every deploy and every traffic burst, the first requests to our booking API timed out and Postgres started refusing clients. The handler did heavy setup and opened a fresh connection per invocation. Moving init out of the handler and adding RDS Proxy fixed it.
A Lambda function that converts uploaded videos to thumbnails worked for the first few invocations, then failed with No space left on device. Files written to /tmp survived across warm invocations of the same execution environment and filled the 512 MB default. How we found it and fixed cleanup, sizing and streaming.