Tracing Paper Over Nav Charts -- A Lesson Re-learned

The Royal Navy this week released the results of the official inquiry into the grounding of HMS Trafalgar (S-107) while doing Perisher PCO training back in 2002. The news reports are focusing on the use of tracing paper to cover the chart of the local oparea, which apparently obscured some information that could have prevented the grounding had it been more noticeable. An excerpt from one report:

A group of officers were in the last stage of a command examination nicknamed "The Pressure Cooker", in which they control the submarine during a simulated attack.
They had submerged and trainees were estimating their position from previous track and depth readings.
The tracing paper was put over the chart so they could draw their course on it - but the inquiry found it had obscured vital information.
This included symbols showing the strength of the current, which led them to misjudge their position.
It also hid part of the contours of the sea floor, which they were using to judge when it was safe to turn.
This led navigators to change course too early and head into water where the seabed was rising sharply.
The inquiry criticised the skipper and senior officers for not monitoring the sub's position separately using all navigational aids. It also recommended a ban on the use of tracing paper to overlay charts.
It said: "The chart became increasingly untidy and elementary mistakes were made.

To me, this wasn't really a "training aid"-induced accident; I think all boats used tracing paper over charts when you were staying in the same small area for a long time with lots of maneuvering, whether you were doing PCO Ops or not. All submariners, I'm sure, have good stories about where training aids actually contributed to real-world casualties; unfortunately, all of mine happened back aft, so I can't talk about them. If you have any that pass the NNPI test, let's hear about 'em in the comments.