Using the JIRA 2.0 integration, if you try to link to an existing record type that is not mapped in the integration config, the error message is very unhelpful. It just says "system error - contact Aha support if this persists" (or something like that). There is no helpful message at the time saying this is not a correct record type nor is there anything in the integration logs.
So for example, I have an integration in my product that maps features in Aha to Stories in JIRA. If I try and link an existing Aha feature to an existing Task in JIRA, I get the above error. It took us some time to work out what was wrong as Aha was not telling us what the problem was. We eventually worked it out, added a new mapping and retried - it worked fine.
Better error messaging in this situation would have enabled us to resolve it much more quickly.