Let's say something like this is being returned.

400 Bad Request: The browser (or proxy) sent a request that this server could not understand

I once got this when attempting to submit a POST request to my Flask app. For example, my Flask App had a route like this that expected JSON.

@views.route('/api/v1/example', methods=['GET', 'POST'])
def apiv1example():

    try:
        json = request.get_json()
    except Exception as exception:
        print(exception)

And I had a Python script making a POST Request to my Flask app. 

#!/usr/bin/python
import requests

url     = "http://www.example.com/api/v1/example"
payload = { "hello": "world" }
headers = { "Content-Type": "application/json" }

requests.post(url, data=payload, headers=headers)

This issue was that request.post had "data" instead of "json".

requests.post(url, data=payload, headers=headers)

I simply had to just change "data" to "json".

requests.post(url, json=payload, headers=headers)