Ha! I knew I wasn't the only one.

I'm a large user of AWS but, in general, I never feel like I'm in a true VPS. My last experience:

Out of the sudden one of our EC2 instances could not connect to another, causing our HA solution to spun dozens of instances and eventually crash too. It was clear to me that the dest machine was behind some firewall, we went to the security group, the machine was supposed to accept any connection, from any port, any host. The instance itself had no active firewall.

Out of desperation I added the very own security group to itself. It worked for a few hours, then stopped again, I removed, it came back to work and still working (8+ months now)

This is only one of various mysterious events I've seen happening on AWS.