Yes, similarity can be found across classes. Whether goods are similar is not determined solely by class numbers: similar goods expressly cross-referenced across classes in the Classification of Similar Goods and Services, and goods closely related in function, use, sales channels, and consumer base, can still allow a prior trademark to serve as a ground for rejection even though they sit in different classes.
A Class Is Not a Firewall
Many applicants assume that "if I register clothing in Class 25, someone else's luggage registration in Class 18 has nothing to do with me"—that is a misconception. A large number of goods in the Classification have cross-class similarity relationships: for example, certain foodstuffs are cross-referenced as similar across classes, and clothing is closely related to certain clothing accessories. When judging similarity of goods, examiners look at how related the goods themselves are; the class is only a reference.
Which Situations Carry High Cross-Class Risk
First, goods for which the Classification expressly notes cross-class similarity; second, goods with identical or complementary functions that are often sold together, such as coffee and coffee makers; third, a raw-material/finished-product relationship; fourth, cases where the prior trademark is highly renowned, which broadens its scope of protection and raises the probability of a cross-class similarity finding or even cross-class protection as a well-known trademark.
Between Goods and Services
A trademark for goods may also be found similar to a related service—for example, tea beverages in Class 30 and teahouse services in Class 43—because the public would readily assume they come from the same business entity.
Self-Check Suggestions
When searching, do not limit yourself to your own class; use the Classification's notes to identify related classes and search them together. When planning your layout, also bring related classes within your defensive perimeter, so that no one else can squat in a related class and turn around to restrict you.
If you encounter a similar situation, it is advisable to first consult a record-filed agent on MyTMBee to assess whether a trademark registration assessment is needed before deciding on the next step.