# The error `slot.collision`

**internal**: a check on MIME's own construction, which no supported script should reach: a MIME defect, to report with the file the error names; or an error MIME raised with no code yet (error.uncoded), keeping its own class, to report so that it gets one.

**On the refusal list: no test names this code**, since no file under `tests/` names it. A test may still raise it without naming it, and none asserts it: what its message and action say is read from the source below. The hook that records which codes a test raises has not landed, so until it does the list says names and never raises. The list (`docs/ratchets/refusal_codes.txt`) only shrinks.

Where MIME gives it, with what its message says happened and what its action says to do (`{…}` is filled in from the experiment):

- in `mime/build/_plan.py`:
  - what happened: `field {…}: readers {…} and {…} map to one slot name {…}`
  - what to do: `MIME defect: report it with this build report.`

Reading an error and reaching its full traceback: [reading an error](../reading_errors.md). Every code: [the index](index.md).