Single Point of Contact (SPOC) is a way for organizations to narrow the possibility that critical information might go to the wrong place.   It can be a person or a department.

A help desk for technolgy support is a good example of this.  A central help desk can ensure that issues (perhaps with a common root cause) are collected, recorded and initially triaged together.

This avoids multiple groups from working on the same problems in isolation of each other potentially causing rework or duplicate work or just confusion.

Single Point of Failure

A help desk can use special applications to track issues, automated, voice and email response together with resource scheduling to help ensure incoming issues do not get overlooked and enough resources are on hand to cover the volume of issues.

In the case of a single point of contact being an individual this is not the case.  Take for example the case where a specific skill and expertise (e.g., specialized business or technology knowledge) is limited to a single source.

Initial contact needs to flow through this source.  In these cases the benefits of SPOCcan be undermined by the fact you now have a single point of failure.

This can become a road block to responding to incoming requests, cause errors due to inadequate capacity to handle too high a volume of contacts and of course the biggest issue might be if the SPOC gets hit by a bus.

Is there someone with enough knowledge to fill in for them?

The bottom line is if you are using a SPOC be very mindful of, and mitigate against, the risk of having a single point of failure.  Typically is the risk occurs the results are not optimal to say the least.