I previously ran into this as well. Tasks create a querystring to note comments, etc, so any limitations to the length of a URL come into play. See MS KB article 208427 for more info.

My issue was I created a Task to "Copy" all an existing incident's fields (but not the entire comment history, only the last comment entered) and make it a child of the source incident. After all the other field info was entered into the querystring for the task, I figured I would only have about 1,100 characters left as the maximum that the last comment could be. Otherwise, invoking the task would result in a "Page Cannot Be Displayed" error.

My workaround for this was to educate my workers as to this limitation and to have a tooltip for the Task link that told the worker the task would error out if more than 1,100 characters were used for the last comment entered. Not the most elegant solution but it works for us and if they run across this they know to cut down the initial comment and then edit the newly created incident and put any additional info into the next comment version.

If someone has a better workaround please let us know. Thanks

 Thanks for the response.  I think we have a bit more control over the situation because the parent incidents are automatically generated.  However, this is a pretty silly situation.