Years ago a lot of these personal techniques were common in my experience. Folks would add knots to the approach speed for the wife and kids. Non-standard additives would be made for wind with autothrottles in use.
Seems like some of the Air Force guys would brief 'I'm going to duck under a dot on short final since the viz is good' while landing a widebody. Was it taught in the T-38, the C-141 or the C-5 perhaps?
One of the regional carriers decades ago had a non-approved B-737 short field technique of pulling up the speedbrake handle before touchdown so that the boards would fully deploy the moment there was weight on the wheels. When a newer version of the 737 arrived, ground spoiler mode was triggered by radar altitude, the boards fully deployed before the wheels were on the runway and a hard landing mishap occurred.
In recent years, thankfully, most of these cowboy techniques seem to be less common where I've worked. However, with all the airline 'mergers' I'm sure non-standard 'we did it this way at Brand X' procedures will give the feds and the training department a lot of job security for years to come.