sum product if three fields are true

Given the size of the matrix - pre XL2007 you should really add a key to Errors sheet, eg:

Errors!L2:
="@"&A2&"@"&B2&"@"&I2&"@"
copied down for all rows

You can then use COUNTIF which is efficient whereas your alternatives are not.

Production!C6
=COUNTIF(Errors!$L:$L,"@"&$B6&"@"&LOOKUP(REPT("Z",255),$C$2:C$2)&"@"&C$3&"@")
applied to matrices

In XL2010 (as in XL2007) you could use COUNTIFS to negate need for Key

Without the key you are left with Arrays and/or SUMPRODUCT which are inefficient and should not be used in large quantity (as would be the case here) - assuming performance is of interest.

Thanks for your assistance day92, appreciate it.

Originally Posted by DonkeyOte

Without the key you are left with Arrays and/or SUMPRODUCT which are inefficient and should not be used in large quantity (as would be the case here) - assuming performance is of interest.

You really are a smart cookie, I aspire to pick up this knowledge...performance is always of interest and I am afraid that is one aspect that I do not fully understand...volatile excel functions...someday I'm hoping I will

Originally Posted by 00Able

I aspire to pick up this knowledge

I'd draw your attention to the below:

Originally Posted by Rita Mae Brown

Good judgment comes from experience, and often experience comes from bad judgment

That's how most people learn that SUMPRODUCTs & Arrays are bad news when used in large quantity and/or with large precedent ranges and/or in Volatile manner.

Originally Posted by 00Able

performance is always of interest and I am afraid that is one aspect that I do not fully understand...volatile excel functions...someday I'm hoping I will

Check out the Volatility link in my sig. - and in fact read through the whole site in full - it's the best resource on Excel performance & optimisation re: calculation engine etc...
(the author Charles Williams is an MS Excel MVP and has written white papers for them on the same subject - covering a lot of the same material)