Home for the Holidays - S11-E1
Other mistake: Only one set of prints were said to be on the garrote but in the last episode of season 10 Graham handles the garrote before handing it over to the murderer without wearing gloves. So there should have been 2 sets of prints.
Continuity mistake: After the failed attempt to stop Mr. Dilbert's bleeding, the ambulance attendant collects and takes away all the bloody rags they used for hemostasis. Still, the very next cut shows a whole pile of rags on Mr. Dilbert's body. To make the mistake even more evident, both his shirt and these rags are all clean with no blood on them. (00:42:35)
Continuity mistake: When Murdoch and Crabtree arrive to the fairground to apprehend Agnes, they lean their bikes against the "annual fair" banner stretched on a handrail. The camera shows the detective and the constable from behind, making visible that the weight of the bikes make the banner to ruck down significantly. However, when the next cut shows them frontwise, the banner is in neutral position. (00:40:30)
Answer: It's certainly a personal choice about using one, and probably similar to how some people refuse to wear a helmet or forego other safety equipment. Most bells and horns on bikes are not very loud and probably wouldn't be heard in busy traffic, making them mostly ineffective. Murdoch would likely still yell, even if he had a bell or horn.