This is a really good question.
The most simple error that is very common is NOT answering the actual tender question. Let’s take an example
Q: Please describe your approach for implementing this…
The ideal answer would describe the background eg WHO would be involved, and what their individual backgrounds (what are there key skills, and why you have selected them for this offering. Plus WHAT you will do (like how you will actually do the service / carry out the services) The ideal answer will include covering some of the risks such as transport / logistics /IT etc.
Top Tip: Consider how to display this. This could be displayed really well on a flow chart showing each stage that will be covered. It is important to consider timings and make sure that you cover all the activities as part of your approach answer.