Youtube API - limit without developer key - youtube-api

Question

For a client of ours, we use the Youtube API v2 to fetch the newest video with a specific tag. We fetched the response of the URL without a developer key. Up until a few days ago, this worked perfectly. Suddenly, we got no results back. Instead of getting an error, we just got 0 results.
After a bit of puzzling, we found out that this is because we probably went over a limit of requests. This is a conclusion based on the fact that when we went to the same URL with a mobile device on 3G as a laptop connected with WiFi, we got results.
My question is this:
Is it possible to get an error in our request if we went over a limit?
What exactly is the limit when not using a developer key?
Is there a premium account available to fetch a much bigger amount of requests?
Thanks in advance!
[EDIT]
When we request the URL https://gdata.youtube.com/feeds/api/users/AEGON/uploads?category=2012Q4&alt=json, we get the following response in JSON:
(I removed some links because of reputation on Stack Overflow)
{
"version": "1.0",
"encoding": "UTF-8",
"feed": {
"xmlns": "",
"xmlns$openSearch": "",
"id": {
"$t": ""
},
"updated": {
"$t": "2013-03-12T08:35:47.226Z"
},
"category": [{
"scheme": "",
"term": ""
}],
"title": {
"$t": "Uploads by AEGON",
"type": "text"
},
"logo": {
"$t": ""
},
"link": [{
"rel": "related",
"type": "application/atom+xml",
"href": ""
},
{
"rel": "alternate",
"type": "text/html",
"href": ""
},
{
"rel": "",
"type": "application/atom+xml",
"href": ""
},
{
"rel": "",
"type": "application/atom+xml",
"href": ""
},
{
"rel": "self",
"type": "application/atom+xml",
"href": "https://gdata.youtube.com/feeds/api/users/AEGON /uploads?alt=json&start-index=1&max-results=25&category=%7Bhttp://gdata.youtube.com/schemas/2007/keywords.cat%7D2012Q4"
}],
"author": [{
"name": {
"$t": "AEGON"
},
"uri": {
"$t": ""
}
}],
"generator": {
"$t": "YouTube data API",
"version": "2.1",
"uri": ""
},
"openSearch$totalResults": {
"$t": 0
},
"openSearch$startIndex": {
"$t": 1
},
"openSearch$itemsPerPage": {
"$t": 25
}
}
}

Answer 1

Answer for your questions,
you'll receive an HTTP response with a code of 403 and a response
body that includes
[<errors><error><domain>yt:quota</domain><code>too_many_recent_calls</code></error></errors>]
for excessive request.
There is no such limit, you need to stop the request for 10 to 15 min. after receiving error.
No idea for this.
Reference: http://apiblog.youtube.com/2010/02/best-practices-for-avoiding-quota.html

Answer 2

According to youtube documentation, you will receive an error message letting you know that you exceeded the quota.
Check Operation limits