At the end of the song "Dirty Town", a sustained chord is heard on the guitar which is punctuated by the guitarist rapidly switching the guitar's toggle pickup selector. This effect is impossible to achieve using the Yamaha Pacifica guitar that Luke Shand plays in the film.
On their way from Groningen to Antwerp the band is passing by some windmills in some typical 'polder' landscape. That sort of landscape with that sort of windmills can not be encountered anywhere along that route, while the possibility of the band having terribly strayed off, gets contradicted by the fact that the two hitchhikers are also heading for Antwerp.
During their Dutch tour, when Ray is being interviewed, we see a Dutch sign behind the band members which reads: "Verboden te parkeren" (No Parking). But the sign above the men's toilet next to the sign reads "Herren" which is German for "Men" instead of the Dutch word "Heren".
In one of the last performances, Luke asks Tony what key they're in and Tony shouts "E and B." But the chords actually being played in that part of the song are A and D. Could be another band foul-up, but at this point in the film, things are supposed to be coming together.
After the disastrous Dutch concert where Ray goes missing, the rest of the group drinks a beer at the bar. Hughie is drinking Grolsch, while the rest appears to be drinking Brouwers Bier. The latter is an "economy" supermarket label, and extremely unlikely to be served in normal Dutch establishments.