Foundation-model agents are being deployed faster than the community can characterize how they fail. In barely two years they have moved into browsing, software engineering, computer use, scientific discovery, and embodied control, where reliability is governed by long-horizon, closed-loop interaction that current single-turn evaluation paradigms rarely capture. Failures (error cascades, reward hacking, mode and reasoning collapse, memory drift, and over-reasoning loops) still lack a shared vocabulary, reproducible triggers, and comparable diagnostics. This cutting-edge tutorial gives a systematic, mechanism-first account of agentic failure modes across four axes: (1) environments and infrastructure, (2) training dynamics, (3) evaluation and red teaming, and (4) test-time scaling and adaptation, with safety threaded throughout. Attendees leave with reproducible triggers, comparable diagnostic protocols, and verifiable mitigations.