MI This condition is triggered when an error occurs. You should use the Error string (for on error) expression to get the error in string format in the same event.
Every frame you have Lacewing in should have an On Error event. This is even more vital with Blue, not due to more bugs, but due to it being more consistent than Relay with reporting improper use of the object.
For example, Relay will not report a single error if you keep setting your name to "Bob" in an Always event while connected, but Blue will report that the name is already set to Bob the second time and further.
Be aware some actions can cause two or more errors, so make sure you combine your error messages together when you display them, instead of only showing the most recent one.
There are many things that can cause an error, but there has been no official complete list.