For example, a mistake would be to buy a Microsoft Excel licence because you want to store data that should be made accesible to web clients through SQL-queries, as Microsoft Excel is not designed for that purpose. In other words, you choose a wrong method for achieving your objective. However, if you installed a Postgresql Server for the same reason but in your haste forgot to give the programme privileges to go through your firewall, that would be a slip. You chose the right method of achieving your objective, but you made an error in carrying out the method.
Both Reason (1990) and Norman (1988) have described several kinds of slips (see 'related terms' below). According to Sternberg (1996), "slips are most likely to occur (a) when we must deviate from a routine, and automatic processes inappropriately override intentional, controlled processes; or (b) when automatic processes are interrupted - usually as a result of external events or data, but sometimes as a result of internal events, such as highly distracting thoughts." See the glossary term Capture Error for an example.
Overall, it should be noted that "The designer shouldn't think of a simple dichotomy between errors and correct behavior: rather, the entire interaction should be treated as a cooperative endeavor between person and machine, one in which misconceptions can arise on either side." (Norman, 1988: p. 140)