"Six minutes of madness" were Guernsey FC's downfall as they lost 3-1 to Horley Town FC in a friendly fixture at the New Defence in Surrey on Wednesday.
The Green Lions went ahead in the first half through Ross Allen.
But in the second half they failed to deal with set-pieces into the box and the home side capitalised.
Assistant Coach Colin Fallaize said: "We certainly more than held our own, we dominated, but those mad six minutes cost the game."
He said: "We actually had an excellent performance, played extremely well, but some lessons to be learnt from this if we're going to come away with three points when we visit the UK."
Amongst the crowd was the new club's president, former England and Southampton striker Matt Le Tissier.
Guernsey FC's first competitive fixture is in the Combined Counties League Division One on Saturday, 6 August against Knaphill at Footes Lane.