In MySQL 5.0, the database no longer keeps an extra byte available. You may need to increase the total size of your DECIMAL column types by one to fix this change.
For example, if you used to have a decimal of “4,2”, this would allow up to 5 characters, not 4, under MySQL 4, because MySQL 4 kept an extra space available (for a leading plus sign). But under MySQL 5.0, only 4 characters are allowed: two on the right and the remaining two on the left. If your data contains decimals with more than two digits on the left, you’ll need to change this to “5,2”.