In "border cases" you might find interesting CEF adjacencies in your CEF adjacency table (displayed with show ip cef adjacency). Most common one is the glean adjacency used for directly connected routes (this adjacency type is a placeholder that indicates the router it should perform the ARP table lookup and send the packet to directly connected neighbor). Discard, Drop, Noroute and Null adjacencies are obvious, the "weird" one is the Punt adjacency, which indicates that the router cannot CEF-switch the packet toward the destination (due to a feature being used that is not yet supported by CEF), thus the packet is punted to the next switching method (fast switching and ultimately process switching).
It's very easy to create a punt adjacency with a back-to-back X.25 link, for example
a1#show running interface serial 0/1/0
ip address 172.16.4.1 255.255.255.0
encapsulation x25 dce
x25 address 1
x25 map ip 172.16.4.2 2
a1#show ip cef adjacency punt
Prefix Next Hop Interface
172.16.4.0/24 attached Serial0/1/0
You can inspect the "special" CEF adjacencies in your router with the show ip cef adjacency discard¦drop¦glean¦noroute¦null¦punt command.