Are very slight errors in garment sewing—such as an increase or decrease in stitches—considered unlawful (haram), and is it necessary to inform the employer about them?

Unintentional errors are not sinful. If the error is minor during tailoring, such as a slight defect that can be remedied and does not affect the required garment specifications, then no guarantee is necessary, especially for defects the size of a finger or similar. However, if the defect is larger, then the tailor is liable for compensation.