NEVER write some random piece of bare data to common (system-wide, site-wide, etc.) logs, e.g. in PHP using error_log(), even during “quick” debugging: ALWAYS include some kind of tag identifying the piece of data and why it is being logged.

If you get interrupted and forget you put that logging in, and don’t discover it in the logs until a week later when that task had long since gone out of your mind, you could end up spending easily half an hour chasing the source of the “problem” down.

Never work for free. There’s a near 100% chance the recipient will be ungrateful… and you know how I know they won’t be grateful? Because if they cared about your work, they wouldn’t have asked you to do it for free in the first place.