I have been charge an extra $40 for using 333MB of data more than my plan limit. However, I only received a text that warned me at the 90% limit and never a text that warned me at 100%. This seems like a scam and shouldn't be happening.
The Data Tracker notifications are only sent after your internet/data session has ended. If you use all of your data as well as some overage usage in a single session, you would still not receive the notification until after the session ended.
It has always been up to us to monitor our own usage. Depending on the phone you have you can set data limits. Android phones have a data tracker and the ability to set limits. Once you correctly set your billing cycle date and a limit, it would automatically turn off the data on the phone.
As far as I'm aware iPhones do not have a similar method of setting limits.
Note: there might be a slight discrepancy between how your phone and the networks monitor data.
I always set my limit 500mb lower than my actual limit.
You can also use the Fido app to track your data usage.