What does it mean if I get an error code for a slip surface?

Error codes are generally not critical and the analysis may be fine even if there are error codes generated. In fact, most analyses generate these codes for some of the surfaces. They are warnings that some of the surfaces could not be computed for various reasons (usually geometric).

In general, we don't use the number of invalid surfaces as a measure of whether the analysis is accurate. It really depends on the code generated. If you have a lot of -111 errors, which is the non-convergence code, then you should look into this in more detail (there are many other codes of this type). If the invalid surfaces lie in a region of interest it is even more important to investigate the results in detail.

Codes are also important for tracking down why an analysis may not generate any results or few results only in a particular area. They should always be looked at, but we would not put a limit on the number. 

Descriptions of each error code can be found in the Summary of Error Codes topic in the Online Help.