You should read the Google Static Maps Usage Limits
The Google Static Maps API has the following usage limits:
25 000 free static map requests per application per 24 hour period.
Additional image requests can be purchased on a per application basis
at the rate currently listed in the FAQ. Additional quota is purchased
through the API Console and requires the use of an API key.
If a user exceeds these limits, the server will return an HTTP 403
status and display the below image to indicate that the quota has been
exceeded:
25 000 requests per day does not seem to be the only limit though. It seems that google does deny access if requests per 15 minutes exceed some 100 or so for a single IP Address. The image you see obviously is a result of these limitations. Usually the limit will be reset soon (after 15 to 60 Minutes).