Item does not match schedule line (program error)

Message no. V1 427

when the transaction code VA02 or VA03 is used the change or displat a sales order the above error message appears.

after analysing through the tables of VBAP and VBEP I see a mismatch in the POSNR.

for example: in VBAP - Posnr 360 is missing and in VBEP - Posnr 370 is missing.

Client is on SAP R/3 - 4.6 c.

Please help me in resolving the issue.

Thanking you in advance