What’s 1 SOP you wish you wrote sooner, and what did it fix?

Context, I'm an agency owner... but have been in the deep world of SOP creation for what seems like 2 years now.

It's been life changing creating simple yet detailed standard operating procedures for employees to follow.

It's slow and boring work, but its what makes the money.

One massive one for me was an "end of day report" which was a simple report employee's filled out allowing me to get a 10,000 foot view on my business, giving me confidence in scaling the agency.

Has there been an SOP in your company that has had a massive impact on either your customers results, or taking a pile of work off your plate.