When inputting a vendor bill, if any of the line items in the bill have the "taxable" field set then when the bill is posted an incorrect entry in the Accounts Payable account is set up, and the bill is incorrectly marked as "paid". Follow the procedure below to reproduce this.

"I'm in the process of becoming VAT registered (in the UK) and I'm
reworking my gnucash accounts to cope. I've created VAT input[A] and
output[L] accounts and created two tax tables: VAT input and VAT output
to refer to them.

I then go to Business -> Vendor -> New bill and create a bill for some
goods (Office Supplies) I've just purchased. I enter the net amounts for
each line item, assign them to Expense:Office Supplies and mark them as
VAT input. Total net value is £31.46. Gnucash calculates the VAT
correctly (give or take a rounding error). I then post the bill, and
this is where things start to fall apart. A zero total value split is
posted to the Accounts Payable account with split lines

VAT:Input
Expenses:Office supplies       31.46
Liabilities: Accounts Payable
Imbalance-GBP                           31.46

When I then try to process the payment it is already marked as paid.

If I then go back, unpost the bill, mark each line item as not taxable,
and re-post it, everything works as it should. The Accounts Payable
account has a £31.46 split with two lines in it

Expenses:Office supplies       31.46
Liabilities: Accounts Payable           31.46

When I go to process the payment, all works fine.

I've reproduced this with a brand new set of business accounts (created
new file, ticked the "Business Accounts" item in the Account Druid) and
the actions above.

I'm running svn r14212 on Ubuntu 5.10, but I've seen identical behaviour
on Fedora Core 3"