SQL server error - string or binary data would be truncated - even though SELECT statement returns nothing -

i have written query in stoted procedure, like,

insert table1     (uniquestr, col1, col2) select uniquestr, col1, col2     table2 ... 

it gives me error:

string or binary data truncated.

here statistics.

1. table1.uniquestr varchar(11)
2. table2.uniquestr varchar(20)
3. table2 has records having uniquestr values of 11 characters , 15 characters.
4. the clause of query written in such way select statement never return records having uniquestr length greater 11.

the first weird scenario - though select statement returns nothing (when run separately), gives truncation error when run along insert (i.e. insert...select).

second weird scenario - gives error in production environment. gave no error in uat environment. in production environment, ran fine 1 day.

can tell me issue?

note: fixed error using substring function not find out reason why sql server gives error?