Current tombstone + flash back strategy cannot handle the following rare case:

• Alter table by widening a column
• Insert a row with the widened column's value to be out of the range of the type before widening
• Drop table
• TiFlash does a full schema sync and directly tombstones the table (with the alter diff missed)
• TiFlash flushes the new inserted row into storage

In such a case, TiFlash won't be able to decode this row and errors out "overflow detected".

But as you can see, this is a extremely rare case.