You can view such warnings before report preview:
Group related band(s) visible, but the corresponding grouping expression not assigned
Group Header band(s) or Group Footer band(s) are visible, but the corresponding grouping expression not assigned. In this case, the group related bands of the corresponding levels will not be printed. For example, Group 2 Header or Group 2 Footer are printed only if [Report].Grouping Expression 2 was assigned.
Grouping expression assigned, but grouping features never used in report
Some of grouping expressions was specified, but Group Header band and Group Footer band of the corresponding grouping level are invisible and the grouping level never used in Expression objects. For example, you specified [Report].Grouping Expression 3, but forgot to set [Group 3 Header].Visible and [Group 3 Footer].Visible to True and never used grouping of the third level in your Expression objects. Please note, that specifying grouping expression will cause calculating its result for every database record in the report, and thus will slow up the report generation.
Object '<ObjectName>' refers to unknown field
Object of Database Text, Database Image, or Database Checkbox type refers to unknown field (or the field is not assigned). You should specify the correct field name.
Object '<ObjectName>' does not contain picture
Object of Image type does not contain a picture (the picture not assigned).
Object '<ObjectName>': expression not specified
Object of Expression type does not contain an expression.
Object '<ObjectName>': text not specified
Object of Label type does not contain text.
Object '<ObjectName>': file name not specified
The corresponding property of File Text, File Image or File Icon object does not contain a file name.
Object '<ObjectName>': file does not exist
Object of File Text, File Image or File Icon type refers to unknown file.
'<ObjectName>': function '<FunctionName>' is not supported
Object contains unsupported function in some of its properties. Please refer to the full list of unsupported functions with a guide how to replace them with current equivalents.
'<ObjectName>': function '<FunctionName>' is obsolete
Object contains obsolete function in some of its properties. Please refer to the full list of obsolete functions with a guide how to replace them with current equivalents.