Unstitching the Chimera: Frame-Level Risk and Train-Free Mitigation for Video Hallucination

Hallucination limits the reliability of multimodal large language models (MLLMs), and it is particularly damaging in video where errors manifest as distorted narratives rather than single-frame mistakes. We introduce a frame-first study of Chimera Hallucination: model stitches visual segments that exist in space and time but do not belong to the same event chain, producing a spurious continuous story. We introduce CH-Risk, a single-forward, reference-free risk estimate tailored to this failure mode. CH-Risk combines two complementary signals: SegCoverage@\alpha (\mathrm{SCR}@\alpha\) measures how many event segments are needed to cover most text-to-frame support, exposing long-range stitching; Alignment with Early Temporal Pathway (AETP) measures rank consistency between support and the temporal pathway formed in early–middle layers, exposing stage mismatch. To turn risk into correction, we further propose CH-M(itigation), a train-free two-stage intervention. Segment-aligned Stage-Aligned Frame Routing (sSAFR) re-weights frames before the mid-layer softmax to route attention toward a small set of pathway-aligned segments. Residual Token Calibration (RTC) then stabilizes token usage within selected segments. Extensive experiments across 9 benchmarks and 6 VideoLLMs show that CH-Risk can predict Chimera and that CH-M consistently reduce hallucination and improves task accuracy with negligible overhead (sub-5% latency, sub-2.5% memory, $$1% FLOPs).