ObserveTestIsolateResolve

Public IT incident case studies / Vol. 01

Don’t stop at
the first answer.

Root Cause Casebook is a field guide to how technology teams diagnose public IT incidents. Each evidence-grounded case preserves the hypotheses, wrong turns, decisive tests, and reasoning that narrowed the fault domain.

Study the latest case
Current case

Applications · Complexity 7/10

Resend outage on February 15, 2026: database connection exhaustion across mixed deployment patterns

Starting at 10:19 PM UTC on February 15, 2026, Resend experienced email sending delays and an inaccessible dashboard caused by database connection exhaustion. Idle connections were not released quickly enough under load, and one misconfigured service spiked from about 60 database connections to more than 330 without a matching traffic increase. Email delivery continued in a degraded state because the sending platform tolerated database unavailability, but most messages were delayed by roughly two hours and dashboard and non-email API operations were disrupted. Full recovery was reached at 1:50 AM UTC after connection pool and role-level database limits were tightened.

How to investigate an IT incident

  1. 01Symptom

    Record what users and systems actually reported.

  2. 02Hypothesis

    Keep the first plausible explanation provisional.

  3. 03Evidence

    Find observations that narrow the fault domain.

  4. 04Turning point

    Identify what changed the investigation’s direction.

  5. 05Root cause

    Name the mechanism that survived the evidence.

Explore the troubleshooting field guide →

Previous investigations

Which incidents are in the Casebook?

Full case archive →
  1. 02
    Supabase us-east-2 Regional Outage: Monitoring-Service Deployment Accidentally Enabled VPC Block Public Access Region-WideFebruary 12, 2026 · Networking
    6/10
  2. 03
    Redocly January 2026 service disruptions from orchestration-layer instability and queue retry cascadeJanuary 27, 2026 · Cloud Infrastructure
    8/10
  3. 04
    Cloudflare outage on November 18, 2025: ClickHouse permissions change doubled Bot Management feature file and panicked the core proxyNovember 18, 2025 · Cloud Infrastructure
    8/10
  4. 05
    Claude Infrastructure Degradation Triple-Bug PostmortemSeptember 17, 2025 · AI & Automation
    9/10

Questions

Questions about the Casebook

What is Root Cause Casebook?
An evidence-grounded field guide to how technology teams diagnose public IT incidents. Each case preserves the hypotheses, wrong turns, decisive tests, and reasoning that narrowed the fault domain.
Which incidents does the Casebook cover?
Public technology incidents with enough public evidence to reconstruct the investigation: cloud outages, AI infrastructure degradations, and major service disruptions. Cases include the May 2026 Azure OpenAI multi-region failures, the June 2026 Google Gemini outage, and the November 2025 Cloudflare outage.
How are the cases researched?
Every case is reconstructed from public sources: status pages, official postmortems, and engineering write-ups. The methodology page documents the evidence standards, and uncertain claims are labeled as such instead of presented as fact.
Is Root Cause Casebook free?
Yes. All cases are free to read with no account required, and new investigations are published regularly.