Why Corrupted Training Data Doesn't Show Up as High Loss

Originally published at ai.bedvibe.studio . There is an assumption almost every practitioner carries without examining it: if your dataset has bad samples in it, the loss will tell you. Corrupted rows spike. Broken files stick out. Sort by per-sample loss, look at the top of the list, and there is your garbage. I believed it too. Two separate failures in my own work say it is wrong, and they fail in the same direction — quietly. The reproducible one: a dataset that cannot be learned While validating trainproof I ran a controlled fault-injection study: one base setup, a Qwen2.5-3B QLoRA, run six ways, three seeds each, eighteen runs total. Every log ships in the repo so the verdicts can be checked rather than believed. One configuration shuffled the dataset's labels into pure noise. The labels no longer corresponded to the inputs at all. This is not a hard dataset or a noisy dataset. It is a dataset that cannot be learned , because there is no relationship left in it to learn. That run reduced its loss by 62%. On its own curve it was textbook-healthy — a clean downward slope, no spike, no plateau, nothing a human or a rule would flag. It was learning nothing useful. It was memorising the statistics of noise, which any sufficiently large network will happily do. From a single run's loss curve it is indistinguishable from a real one. That is where the assumption broke for me. Not "loss is a weak signal for this." Loss is not a signal for this at all, in isolation. The production one, and what I can and cannot prove about it The second failure came from real work rather than an experiment, and it is the one I think about more. Building a text-to-speech corpus of roughly 110,000 recordings, a small number of the files were pure loud white noise. Not corrupted in the file-format sense — they opened fine, played fine, had valid headers and valid duration. They simply contained no speech. Just noise, at volume. They did not surface as high-loss outliers. Being precise about