When you are trying to make incorrect inclusion placement (event inside of task) Stunner have to highlight that your action is invalid. For now Stunner helps you to center your Event inside of Task and when you release your Event will show you error (1).

Steps To Reproduce:
1. Open/Create Business Process with Event and Task
2. Take Event and try to add it inside of Task

Actual Result:
Stunner helps you to align you Event inside of Task, but when you release your mouse you will see an Error (1).

Expected Result:
Stunner will show you that operation is not permitted when you move your Event over the Task.

Error [Class=CommandResultImpl, Type=ERROR, Message=The 'org.kie.workbench.common.stunner.bpmn.definition.BusinessRuleTask' cannot contain the labels ['[all, sequence_start, choreography_sequence_start, StartEventsMorph, fromtoall, to_task_event, Startevents_all, from_task_event]']., Violations=[The 'org.kie.workbench.common.stunner.bpmn.definition.BusinessRuleTask' cannot contain the labels ['[all, sequence_start, choreography_sequence_start, StartEventsMorph, fromtoall, to_task_event, Startevents_all, from_task_event]'].]]