Monitor your sequences

Never let your sequence exceed your column precision. Here is simple way to generate a check..

select ds.sequence_name,
dtc.table_name,
dtc.column_name,
ds.last_number,
dtc.data_precision,
round(ds.last_number/power(10,dtc.data_precision),2)*100||'%' percent_used
from all_sequences ds,
all_tab_columns dtc
where ds.sequence_name like 'SEQ_'||trim(upper(dtc.column_name))
order by percent_used desc, sequence_name;