When you are determining field positions, be alert for tabs in the data file. Suppose you use the SQL*Loader advanced SQL string capabilities to load data from a formatted report. You would probably first look at a printed copy of the report, carefully measure all character positions, and then create your control file. In such a situation, it is highly likely that when you attempt to load the data, the load will fail with multiple "invalid number" and "missing field" errors.
These kinds of errors occur when the data contains tabs. When printed, each tab expands to consume several columns on the paper. In the data file, however, each tab is still only one character. As a result, when SQL*Loader reads the data file, the POSITION specifications are wrong.
To fix the problem, inspect the data file for tabs and adjust the POSITION specifications, or else use delimited fields.