Common errors:

* 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)

* deadlocks