Answer:  Human error is the top reason for un-planned Oracle outages, and you must be very careful to know that instance you are connect to, especially when working from a UNIX/Linux command prompt:

First, make sure to include both the server name and the instance name in the command line prompt.  Some even include the current directory name so they know what directory they are using.  This is done by setting the PS1 environment variable.

PS1=" `hostname`*\${ORACLE_SID}-\${PWD} >"
export PS1

Inside SQL*Plus (for 10g and beyond), you can also customize your SQL prompt to include the user and ID:

set sqlprompt "_user' @ '_connect_identifier > "

Creating color-coded Oracle command prompts

Green is for test, red is for prod. and it easy to know where you are.  Adding a colored background also helps to prevent mishaps when you are in production and think that you are in the test instance:

PS1='^[[0;32m^[[1m${HOST}:^[[1;33m${PWD}:^[[0;32m^[[1m${ORACLE_SID}^[[0m> '
export PS1

In production, you can give the screen a red background, as a warning that you are on a production database.

PS1='^[[0;31m^[[1m${HOST}:^[[1;33m${PWD}:^[[0;31m^[[1m${ORACLE_SID}^[[0m> '
export PS1