i had a small incident yesterday in purley, relevant to this topic. four of us were going from brighton into london, i was at the back, about 30m behind from the third cyclist. there was a long queue of traffic, either moving slowly or stopped. when the cars were moving we were on the inside, going at similar speed around 20kph. when they stopped, slowed down to about 15kph. all of a sudden a car pulls out from a parking spot at the side of the road and i hit the wing mirror and off-side fender (no damage to anybody/-thing).
driver's story was: i was indicating to pull out and the bus (going the same direction) stopped to let me through. i started moving and there you (i.e. me) come from the inside the bus and hit my car! you all cyclists do that!
my story: i filter on the inside, slowed down to 15kph as the traffic stopped and the car pulls out right in front of me. i didn't see the indicators blinking as it was midday and the sun was shining from the back. she pulled out very quickly, probably as a "thanks" to a bus driver who stopped to let her go. she failed to see me as she only looked to a bus driver next to her and didn't look over her shoulder before pulling out. had she not pulled out so quickly, i'd manage to swerve or stop in time and avoid the contact.
While filtering can be done only when it is safe to do so, same applies to starting a manouvre in the vehicle - look in the mirror, over your shoulder(s) and proceed carefully.