Unrecognized cells?

I got:
[notice] Since our last heartbeat, 3 circuits were closed because of unrecognized cells while we were the last hop. On average, each one was alive for 120.333333 seconds, and had 1.000000 unrecognized cells.

What does that mean?

Also, I’m running only an OBFS4 bridge, I’ve specifically got:
ExitRelay 0
… in torrc.

Should I be the last hop?