One off/intermittent error from screened as of the time of submitting this bug:

issue1978 (Darcs3) (PatienceDiff): [Failed]
|
| mkdir future
| + mkdir future
| cd future
| + cd future
| darcs  init
| + darcs init
| ============================= WARNING =============================
| The `darcs-3` semantics is EXPERIMENTAL and new in version 2.16. It is included
| only as a technology preview and we do NOT recommend to use it for any serious
| work. The on-disk format is not yet finalized and we cannot and will not promise
| that later releases will work with darcs-3 repos created with any darcs version
| before 3.0.
| ===================================================================
| darcs.exe: SetCurrentDirectory "..": permission denied (The process cannot access the file because it is being used by another 
process.)

(note this was on Windows)

While it is unclear what the exact reason for the permission error was 
in this case, it is clear that in general a cd ".." (to seek for a 
possible repository we are in) can well run into IO errors. I have 
submitted patch2119 which does that.

> While it is unclear what the exact reason for the permission error was 
> in this case, it is clear that in general a cd ".." (to seek for a 
> possible repository we are in) can well run into IO errors. I have 
> submitted patch2119 which does that.

There is a sentence missing in the middle, saying:

We should catch such IO errors, print a warning with the details, and
move on assuming we are not in a darcs repo.