Spellcheck won't save you on this one, dear friends.
Lightning is the stuff you want to avoid during thunderstorms. Lightning never strikes the same place twice, lightning in a bottle, yada, yada.
Lightening means to make lighter. When Mr. Rewrite delivers a good joke, something that rarely happens, he is lightening the mood.
Lightening also applies to the process in which the uterus descends into the pelvic cavity during pregnancy, alleviating pressure on the diaphragm. Mr. Rewrite remembers this from some Web blurb he read while Rewrite Jr. was on the way. Near as he could tell, this process involved Mrs. Rewrite and not him. But anyway, that isn't the point of this post ...
Here are examples of those paid to know better having trouble with lightning/lightening: