- Split contamination
- The same record, or a near-duplicate of it, present in both training and test. The most common cause of an evaluation score that cannot be reproduced in production, and the easiest to miss because nothing looks wrong.
- Label noise
- Items labelled inconsistently, usually because the guideline was ambiguous rather than because anyone was careless. It sets a ceiling on achievable accuracy that no model change can lift.
- Class imbalance
- A rare class with too few examples to learn from, sitting behind an overall accuracy figure that looks acceptable because the common class dominates it.
- Taxonomy drift
- A label set whose meaning changed partway through collection, so early and late records use the same name for different things. Invisible in aggregate, corrosive in training.
- Distribution shift
- Training data collected under conditions the deployment will not see: one camera, one season, one document template, one writing style.
- Missing provenance
- Records with no trace of where they came from or when. It makes a dataset impossible to audit later and, where the data concerns people, difficult to defend.