Suppose we have two templates: T1 and T2. Template T1 is linked to host H. The problem is that we can create a trigger on T2 that references items from both T1 and T2 (note that on H only items from T1 are present).

A related observation that might or might not be a problem: if we link both T1 and T2 to host H and create a trigger on T2 that references items from both T1 and T2, then on H that trigger will be shown as inherited from T1 (or T2), even though it is present in both T1 and T2.