Clock skew across hosts, timestamps out of order, a stack trace spanning twelve lines, and NaN in a Prometheus gauge. The telemetry shapes that break ingest.
Nested MIME, four charsets in one mailbox, authentication headers that pass and fail, and a bounce that looks like a reply. Test your parser against real message shapes.
Antimeridian crossings, unnormalised longitudes, silent CRS assumptions, and a shapefile that is really five files. Test the cases that break importers.
Extractors fail on codecs they never linked, filenames they cannot decode, and nesting they never expected. Here is how to find out before your users do.