One common pattern when using the Task Queue API is known as 'task chaining'. You execute a task on the task queue, and at some point, determine that you're going to need another task, either to complete the work the current task is doing, or to start doing something new. Let's say you're doing the former, and your code looks something like this:
def task_func(): # Do some stuff deferred.defer(task_func) florb # This line causes an error
I'm sure you can guess what happens here. You successfully do some work, successfully chain the next task, then you encounter an error. Your code throws an exception, and returns a non-200 status code to the task queue, which notes the failure and schedules your task for re-execution. When it re-executes, the whole thing happens all over again (if your error is persistent, instead of transient, like the above).
Meanwhile, the task you enqueued runs. Perhaps it also fails after chaining its next task. Now you have two repeatedly executing tasks. Soon you have 4 - then 8 - then 16 - and so forth. Disaster!
"Ah, " you may say smugly, "I don't do anything important after chaining the next task ...