I've come across a number of issues in the past where the cause of the issue is that someone hand edited the web.config and resulted to a not well formed XML (missing tags, unclosed quotes et al).
The recommended way of doing it is to minimize these issues is to use the ASP.NET Configuration Settings Dialog which is available to IIS (not sure if there is a stand alone executable for this). Here are simple steps to do it.
1. Open IIS
2. Right Click on target site/virtual directory and select Properties
3. The Properties dialog should show up. Select ASP.NET tab.
4. At the ASP.NET tab, you should see and click Edit Configuration. (if you're opening Properties of a website, you should also see an "Edit Global Configuration" button which corresponds to the machine wide web.config). Doing so should display the Configuration Settings Dialog
5. Make your modifications without fear of unclosed quotes whatsoever. You could likely still mess up some settings but I can assure you that it would be easier to do when you're manually hand editing the file.
Also note that when adding a web.config in Visual Studio (using Add New Item) you will get a comment somewhere at the top stating:
Note: As an alternative to hand editing this file you can use the
web admin tool to configure settings for your application. Use
the Website->Asp.Net Configuration option in Visual Studio.
A full list of settings and comments can be found in
machine.config.comments usually located in
The "Website->Asp.Net Configuration option in Visual Studio" mentioned are items in the toolbar and appears if you have the website project (or its files) open/selected (see image below)
This is also a recommended way of doing it for web site projects (not available in Web Application Projects).
Editing web.config is likely inevitable for development but as much as possible I would recommend not hand-edit them unless you (or others in the team) really have to (especially for those with not much experience with XML/HTML - it is trivial but could minimize some headaches)
UPDATE: I have updated the post title from "Hand Editing web.config, not" to "Hand Editing web.config or not" since it seems to make a big difference. I personally hand edit most if not all web.config I work with but this is merely an alternative based on experience where there are actually people out there guilty of messing the xml for a lot of reasons. Being an alternative it may or may not be best for all situations. Also, as a starting/budding developer it is sometimes (if not all the times) best to learn from mistakes so I am in no way against hand editing of files. However like gurus out there, be careful in doing as it would probably hurt more than missing something in HTML. Furthermore for those not yet confident enough to mess up with config manually this might be for you. Although again it's best to learn from mistakes if you can afford to do so since you will surely remember it more. :)