Most forensic science evidence is produced in conditions that do not protect the analyst from contextual information about the case that could sway their decision-making. This article explores how these largely unrecognized threats raise real problems for the criminal justice system; from the collection and interpretation of traces to the presentation and evaluation of evidence at trial and on appeal. It explains how forensic analysts are routinely exposed to information (e.g. about the investigation or the main suspect) that is not related to their analysis, and not documented in their reports, but has been demonstrated to affect the interpretation of forensic science evidence. It also explains that not only are forensic analysts gratuitously exposed to such ‘domain-irrelevant’ information, but their own cognitively contaminated interpretations and opinions are then often unnecessarily revealed to other witnesses—both lay and expert. This back and forth can create a ‘biasing snowball effect’ where evidence is (increasingly) cross-contaminated, though represented, at trial and on appeal, as separate lines of evidence independently corroborating one another. The article explains that lawyers and courts have not recognized how contextual bias and cognitive processes may distort and undermine the probative value of expert evidence. It suggests that courts should attend to the possibility of contextual bias and cross-contamination when admitting and evaluating incriminating expert evidence.