Side condition violated at X = 3.000000Side conditions are "boolean expressions": they can be of any of the forms A=B, A<=B, A>=B, A<B, A>B, A<>B, where A and B are arbitrary numerical expressions, or built up from these forms using OR, AND and NOT.
You should avoid the use of "=" or "<>" with floating-point numbers, because roundoff error makes exact equality unlikely to occur. Thus a side condition "X <> 3" will probably not work; if X is increasing, you should use "X < 3" instead.
To create a side condition, use "Insert Side Condition" on the Edit menu. A blank line appears in the Equations window", on which you can enter your side condition. There can be only one side condition. When a side condition is present, the Edit menu entry changes to "Delete Side Condition". You can also delete a side condition by deleting all the text in the Side Condition line and pressing Enter.