I haven't personally found a real bug in terraform or a provider yet, but I've had to refer to the source many times to figure out what is actually happening. It's always been either misuse on my part, or drift that the provider couldn't resolve.
I still consider it a failure though if it takes looking at source code to figure out what's actually going on -- whether it's vendor or in-house. The ironic and annoying part is it usually takes a deeper level of knowledge to write better error messages, but the people with that knowledge don't have the perception of it being a problem. I fight this battle internally with my own teams all the time. The problem is not getting people to make a change, but recognizing that the message is misleading/confusing/unclear to their users (eg: developers who are not domain experts like them) in the first place.
The entire stack is kind of bad at both logging and having understandable error messages.
You get things like this:
Is this a problem with the actual terraform or passing a variable in or something? Is it a problem with the googleapi provider? Is it a problem with the API? Or did I, as the writer of this, simply forget a field?In complex setups, this will be deep inside a module inside a module, and as the developer who did not use any google_cloudfunctions_function directly, you're left wondering what the heck is going on.