Goals from David Dunn and Alan Judge again exposed Celtic's defensive frailties as Blackburn earned a 2-1 victory at Parkhead.
Dunn and Judge netted either side of a Marc-Antoine Fortune strike as the Clydesdale Bank Premier League side and their Barclays Premier League visitors rounded off their preparations for their respective new league campaigns which begin next weekend.
Sunderland went down to a 3-1 defeat to German side Hoffenheim in their final pre-season friendly at the Rhein-Neckar Arena.
Jordan Henderson was on target for the Black Cats with a free-kick which he curled into the top corner via an upright in the 65th minute.
By then, though, Steve Bruce's men were two goals down, Croatian defender Josip Simunic scoring twice inside the first 30 minutes.
And Prince Tagoe, who played for Ghana at the World Cup, added a third for the Bundesliga side just a minute after Henderson's goal, slotting home from close range.
Wigan warmed up for their opening Premier League fixture at home to Blackpool with a 3-1 win at Dundee United, while the Seasiders fared less well against Scottish opposition, going down 2-1 to Hibernian.