At the moment, Commands don't handle errors in a set way. Some Commands handle errors differently to others. This can produce unexpected results.

Generally, if the problem is not with the Sequence itself e.g. The provided database name or Command Sequence name cannot be found or the wrong number of input values are provided for the input variables, then the word 'Error' will always be returned and the Sequence will never be run. Even if your sequence doesn't contain an Return Command, 'Error' will still be returned.
If the problem is in the sequence itself e.g. A variable is being used for a row number and the variable value is 'hello world' so it can't be parsed to an interger, either the Command will not be run at all, or 0 or 1 will be used.

If your Command isn't running as expected, the best way to correct this is by using a Return Command after a Command to return the result of any Commands you think aren't working. You can then check these with the results you would expect and move up the sequence until you get the result you would expect so you know the next Command is causing the problems.