Yesterday I saw my neighbor's pull back home from spending a week on the river. After they got their boat unhitched I walked over to visit. Sure as shootin', I whacked my shin on the ball receiver and got grease all over the shin of my favorite old pair of pants. DRAT! The wife then told me "liquid toilet bowl cleaner and an old toothbrush!" Home I went and tried it, what did I have to lose? My neighbor also said to immediately pour liquid detergent on the cleaner and wash. So I did... and it worked beautifully... no more greasy stain. And it was easy to do.
That got me to thinking of other stains. One of my white polo shirts has a coffee stain down the front. Hole in my lip! I've laundered that shirt a half dozen times, pour a quart of different cleaners on it and the stain was still there. Out came the toilet bowl cleaner and the toothbrush. A couple of minutes later the stain was gone!
Now, I may not want to use a toilet bowl cleaner on dark blue or black clothing, but if something has an icky old stain on it, I figure it's trashed anyway. So far, so good
Your mileage may vary.