"Your data cannot be analyzed because it represents either a vertical or horizontal line"

When you choose the correlation analysis, Prism 5 will present the message "Your data cannot be analyzed because it represents either a vertical or horizontal line" if all of your X values are identical or if all of the Y values are identical. In these cases, it is not possible to compute either Pearson or Spearman (nonparametric) correlation. 

If you run a correlation on only one data set, the problem is usually obvious. If you run a correlation matrix comparing many data sets against many other data sets, this message will appear in a floating note with Prism 5 when any of the correlation computations are impossible because the X or Y values are identica.  When it appears with correlation matrix results, this message is not helpful because it doesn't tell you which correlation results are impossible.

Prism 6 is smarter. It simply leaves the results blank for the data set pairs where correlation is impossible, and reports the rest. No floating note.