There are several categories of case where an exercise by trustees of a discretionary power may be held invalid1:
- 138.1
    there may be a formal or procedural defect, such as the failure to use the stipulated form of document (such as a deed), or to obtain a necessary prior consent;
- 138.2
    the power may have been exercised in a way which the power does not authorise, for example by the inclusion of beneficiaries who are not objects of the power;
- 138.3
    the exercise may infringe some rule of the general law, such as the rule against perpetuities;
- 138.4