The other possibility (which has happened to me two or three times now) - did you have trouble loading the film into the dev tank?

Sometimes it can get crinkled if it doesn't feed into the spiral properly, this will produce interesting marks on the film when you open the tank afterwards. My last roll of Ilford 3200 lost a couple of frames that way (I'll go back to using the Agfa Rondinax tank and keep the Paterson one for 120 film, the Rondinax is much easier to use for 35mm anyway).