* data type mismatch. For example date is expected but a regular string pops up.
* truncation. For example text is too long.
* code page mismatch. You have the regular latin code page (1252) and suddenly you get those weird German symbols in an address.
* connection failures (you already mentioned this + timeout issues)
* metadata changes (source and/or destination. Can happen unexpected with Excel files)
* primary key/unique index violation (duplicate values)
* bad input file format (for example, no text qualifier is chosen and suddenly the delimiter appears inside the data. Or CR/LF becomes LF as row delimiter or the header is repeated multiple times in the data)
* permission issues
* configuration file is missing and suddenly you are inserting data in the test environment instead of the production environment
* out of memory issue
* database/log file too big (disk is full and SQL Server stops inserting data)
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MCSE Business Intelligence - Microsoft Data Platform MVP