When trying to repair certain parts that were salvaged and then re-installed while still in damaged state, a failed repair (at any repair difficulty level) makes the part in question disappear. This definitely happens with engines and actuators, and never happens with weapons and general equipment (i.e. BAP, ECM etc), and only if a part is salvaged and re-installed, such as after its location is destroyed.
This bug has been fixed for the next release.