Error message strings raised by the pg gem (0.12.0) are of the ASCII-8BIT type.
This can cause an issue concatenating those messages in exception handlers; Rails follows the vulnerable pattern here:
The attempt to concatenate the SQL (likely in the database's encoding) and the error message (ASCII-8BIT) will raise an incompatible character encodings exception IF the error message emitted by PostgreSQL contains some high-bit characters from the original query. A short demonstration script:
In Rails, this causes the real error to be masked (here, an unescaped single quote) in favor of a spurious encoding mismatch.
This could be worked around in the formatting code in Rails, but it seems like emitting ASCII-8BIT is not exactly what is wanted here, so I wanted to open the issue on this side instead. Let me know if this is not the proper place for that.
If this seems like a bug and not a feature:
Possible solutions that seem reasonable to me: detect and properly pass thru the encoding of the error message from PostgreSQL (but I don't know how to do that), or explicitly encode the raised message string as the database encoding type, or at least the setting of Encoding.default_internal. I could try to create a patch and/or pull request for either of these last two.