(The converse is a right-side failure, where even with any reduction the resulting state is safe overall.)
If a wire breaks, or the battery is drained, then the relay will de-energize and the signal will show a red light, which is fail-safe.
This occurred with the Hinton train collision, but investigations soon showed that a wrong-side failure was not the cause.
The 2008 Chatsworth train collision was determined to be human error, not wrong-side failure as a few initially said.
[6] In the United States there is a rule that wrong-side failures are to be reported to the Federal Railroad Administration.