A simple code:
```
player allowDamage false;
player addEventHandler ["HandleDamage", {
systemChat "EH 1";
_this#0 removeEventHandler ["HandleDamage", _thisEventHandler];
}];
player addEventHandler ["HandleDamage", {systemChat "EH 2"}];
player setVelocity [0,0,-100];
```
The above code will only show "EH 1". If the `removeEventHandler` line is commented out, both systemChats will be shown.
This is similar to this issue:
```
_array = [1,2,3,4,5,6];
_newArray = [];
{
_newArray pushBack _x;
_array deleteAt _forEachIndex;
} forEach _array;
```
which will give `[1,3,5]`