Second-generation machines will keep fast-forwarding when they (think they) have a prerecorded cassette but there’s no Table of Contents on the tape.
Prerecorded tapes only have one hole on the bottom side to let the recorder know it’s a DCC (not an analog cassette). User-recordable tapes have more holes which indicate the length of the tape. If the switches in the recorder don’t detect the other holes, the recorder thinks it’s a prerecorded tape and will try to find the table of content (which isn’t there of course). This is only a problem in 2nd generation recorders: DCC300, DCC380, DCC600, DCC450, FW91 etc.
If the problem happens with a prerecorded tape, try cleaning the head.
If the problem happens with a user recordable tape, switch the recorder to service mode and test the switches inside the cassette compartment. The switches that don’t work probably need to be cleaned.
Instructions to switch to service mode are in the service manual. If you don’t feel comfortable doing this, the DCC museum can help.