It happens to me too!
client_credentials trying to get a token using:
https://api.twitter.com/oauth2/token
with consumerKey/secret, it sometimes works and sometimes return 403 :
message - Unable to verify your credentials
code - 99
I think it has something todo with a cap on our servers. we have an application at google app-engine, again all seem to work fine but fail rate is high, almost all are getting the following message:
“
403:The request is understood, but it has been refused. An accompanying error message will explain why. This code is used when requests are being denied due to update limits (https://support.twitter.com/articles/15364-about-twitter-limits-update-api-dm-and-following).
message - Unable to verify your credentials
code - 99
”