I've been down with a killer flu for the last few days, and have been listening to To Say Nothing of the Dog on tape. I've read it many times, but something just stood out to me, cut for spoilers.
During the croquet match, Verity hands Ned a note. When he opens it (in the lilac patch while looking for a croquet ball), it's a letter written by Maude to her younger sister, describing meeting Terrence on the railway station, a meeting Ned has messed up by being at the station himself and spooking Maude's aunt into hiring a cab.
The problem: Verity should not have been able to bring the note back through the net, since it (either the original, an historical document which Verity shouldn't have been able to take, or a copy) couldn't exist in the year Ned is in Victorian England. The net isn't supposed to open up to anything incongruous! She could have written a note herself, but Ned describes the letter, including the handwriting, and why would Verity have transcribed, from memory, the whole letter, when she could have just explained the problem in a few words?
These kinds of things almost always show up in time travel books. I like Connie Willis' in part because she does a really good job of weeding them out, but they still slip through.