MySQL 4.0 => 5.0 - strange...

[Alex Smith]

I've began migration from 4 to 5 MySQL. Since I dont use some specific features which are in INCOMPATIBLE list thought migration would be easy.
Strange thing: a simple SQL query

select purchase.unit,purchase.product,store.date,unit.type,retail_store.date as retail_date
from purchase,unit
left outer join store on unit.type<>2 and store.date=... and store.unit=purchase.unit and store.product=purchase.product
left outer join retail_store on unit.type=2 and retail_store.date=... and retail_store.unit=purchase.unit and retail_store.product=purchase.product
where purchase.unit=unit.code

generates an error: "ERROR 1054 (42S22): Unknown column 'purchase.unit' in 'on clause'".
Note that column `unit` in table `purchase` EXISTS! And it exists in query too!

Maybe there is a conflict between column name and table name `unit`? Maybe conflicts with reserved words (but `unit` is not mentioned there)? Maybe SQL implementation changed? Maybe may SQL mode be set more compatible with 4.0? But setting sql-mode=MYSQL40 (seems most reasonable) does not solve the problem! Does anyone have any idea? ;)