I have been asked to provide some software engineering examples of the common cause and special cause variation from Don't apologize be on-time!

Common cause variation in software development would be things like the size and complexity of Features or Stories or Use Cases estimates. For example, an estimate could be under or over and it is likely that these will vary within reasonably bounded limits. It might also be things like staff vacation, sickness due to common illness like influenza or cold. The amount of staff downtime is likely to be bounded to a number of days per year. Whether or not the current project overlaps with those downtime days is the issue at variance.

Special cause variation would be things like office downtime due to the power outage caused by the 20 year ice storm. Special cause would be your Indian chief architect announcing he will be out for one month unpaid leave to get married. He only gets married once. Therefore it won't happen again.

There are gray areas. For example, some special causes may be seen as common causes under different circumstances. For example, let's imagine that not only is the chief architect Indian but half the team of developers is also Indian and male and single. The chance that one of the them will take a month off to get married during any given project is high. Therefore for a manager with a team of this description, a one man month outage due to marriage is common, not special. Hence, this should be buffered as common cause.

Another example might be that your middleware supplier issues an upgrade which subsequently breaks all of the your code due to a bug. If that supplier was previously reliable then this should be treated as special cause variation. However, it may turn out to be an initial instance of a problem which will become more regular. Once a new pattern of behavior is recognized then such variation should be treated as common cause.