What would happen if I used POST for updating a resource.
If you are the developer then nothing will happen except cofusing the API users and not being in the RESTful best practices.
You can refer to this conversation in order to understand better the difference between the 2 and why it's better to stick with the guidelines.
For example, what would happen if i use GET request to create a new entry in my back end database?
Same answer as above, not in the RESTful spirit neither best practices. But I think that are misunderstanding something in the HTTP protocol though:
GET : is better suited to ask for something.
POST: is better suited to ask for something while providing some information (It goes like this : Hey server! here are some data, and make me some data based on them. This is why it's generally used for updating, because it has a payload, so basically POST = GET + Payload).
Asking if you can use GET to create a new update in a database is kind of non-sense, since GET is generally used just to ask for data. GET does have limited payload information embeded directly in the URL, so you can't separate well the URL from the Payload.