When Mannix is keeping the cabin where Fran lives under surveillance with binoculars, he notices three men approach the nearby propane gas tank. The closeup of his view through the binoculars to show them tampering with the valve mechanism changes to a higher angle shot, even though Mannix himself has not changed his position.
When Mannix approaches Alan Brewer and hands him an envelope, telling him that it's a letter from his ex-wife, the sunlight shining through the envelope shows that there is nothing inside.
Right after Mannix shows Wickersham that the boy is sleeping in his bedroom, he mentions 'Brewer' in a line but his lips don't match. It looks instead like he's actually saying 'Webber', which is the name of another character.
Mannix watches two men rigging a gas line beneath the house and a third man setting a fuse to ignite the gas and cause an explosion. Mannix jumps the third man while the first two finish the gas line and make their escape. Mannix wins his fight and cuts off the fuse, then goes up to the house to call it in. He never shuts off the gas or even removes the hose.