Some motions are required by the various states’ strata acts. Eg. a budget motion is required at an AGM. If it is not sufficiently clearly worded it might not fulfil the requirements of the Act, then what might be ‘illegal’ was the failure of the OC to produce an adequate budget resolution.
Other motions might not be exactly illegal but simply ‘of no effect’ since they attempt to do things that can’t be done. For example, if the Act says a certain class of decision must be decided by (say) an unopposed resolution of a general meeting and a general meeting passes a resolution to delegate that such decisions to the executive committee, then the resolution would simply have ‘no effect’ because the Act takes precedence. If the Act says something has to be done one way, the OC can’t resolve to do it a different way.
I could imagine successfully arguing that a GM resolution is ‘of no effect’ because it was not sufficiently clearly worded to determine what was actually resolved. In a borderline case of a poorly worded resolution it might be obvious that some things were clearly intended to be in scope for the resolution and it might be reasonable for an EC to act on the resolution to that extent. However, it might not be clear whether some other things were authorised (maybe in scope, maybe not), in which case it would be wise for the EC to act conservatively in those areas and propose a more carefully worded resolution to clarify the situation.