Unusual import issues
Spaces
One you would never think of, is spaces which are not spaces in records.
For example, the following record was an account in the Dynamics 365 system.
UK MOD - BSMT
When importing a new opportunity with the above account associated it would fail and not find it. The import file had the following account against the record.
UK MOD - BSMT
Looks the same until you paste it into Word and turn on the non-printable characters option. Ohh what fun.