error dumped into console today during a server restart ....

[Error] Failed to call hook 'Unload' on plugin 'MiniCopterOptions v2.0.36' (InvalidOperationException: Collection was modified; enumeration operation may not execute.)
  at System.ThrowHelper.ThrowInvalidOperationException (System.ExceptionResource resource) [0x0000b] in <fb001e01371b4adca20013e0ac763896>:0
  at System.Collections.Generic.List`1+Enumerator[T].MoveNextRare () [0x00013] in <fb001e01371b4adca20013e0ac763896>:0
  at System.Collections.Generic.List`1+Enumerator[T].MoveNext () [0x0004a] in <fb001e01371b4adca20013e0ac763896>:0
  at Oxide.Plugins.MiniCopterOptions.RestoreMiniCopter (MiniCopter copter, System.Boolean removeStorage) [0x000a2] in <548c4da64cce47d7b4b8fad91101f03d>:0
  at Oxide.Plugins.MiniCopterOptions.Unload () [0x00011] in <548c4da64cce47d7b4b8fad91101f03d>:0
  at Oxide.Plugins.MiniCopterOptions.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00038] in <548c4da64cce47d7b4b8fad91101f03d>:0
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0

still getting this error .normally happens on restarts or unloading the plugin

[Error] Failed to call hook 'Unload' on plugin 'MiniCopterOptions v2.0.36' (InvalidOperationException: Collection was modified; enumeration operation may not execute.)
  at System.ThrowHelper.ThrowInvalidOperationException (System.ExceptionResource resource) [0x0000b] in <fb001e01371b4adca20013e0ac763896>:0 
  at System.Collections.Generic.List`1+Enumerator[T].MoveNextRare () [0x00013] in <fb001e01371b4adca20013e0ac763896>:0 
  at System.Collections.Generic.List`1+Enumerator[T].MoveNext () [0x0004a] in <fb001e01371b4adca20013e0ac763896>:0 
  at Oxide.Plugins.MiniCopterOptions.RestoreMiniCopter (MiniCopter copter, System.Boolean removeStorage) [0x000a2] in <51c923a79e614a1eb94c7a57d0d667e9>:0 
  at Oxide.Plugins.MiniCopterOptions.Unload () [0x00011] in <51c923a79e614a1eb94c7a57d0d667e9>:0 
  at Oxide.Plugins.MiniCopterOptions.DirectCallHook (System.String name, System.Object& ret, System.Object[] args) [0x00038] in <51c923a79e614a1eb94c7a57d0d667e9>:0 
  at Oxide.Plugins.CSharpPlugin.InvokeMethod (Oxide.Core.Plugins.HookMethod method, System.Object[] args) [0x00079] in <d09a1f46ca2f4432811bcfe45ad13c7b>:0 
  at Oxide.Core.Plugins.CSPlugin.OnCallHook (System.String name, System.Object[] args) [0x000d8] in <cf88a28c7fb44d36890d85a78331cc9d>:0 
  at Oxide.Core.Plugins.Plugin.CallHook (System.String hook, System.Object[] args) [0x00060] in <cf88a28c7fb44d36890d85a78331cc9d>:0 ​

copter.children is modified by RemoveChild.

Merged post

@Pho3niX90 you could use a backwards for loop