Episode #1.1 - S1-E1
Revealing mistake: When Michael sets fire to Madge's magazine, you can see that the flame was not directly coming out of the lighter. The effect was added in during post-production. (00:05:15)
Episode #2.1 - S2-E1
Revealing mistake: When Martin and Katy arrive in Altea, Martin is acting like the suitcases are very heavy. The suitcases don't actually contain anything as you can see the wheels bouncing around on the floor, revealing an extremely light case.
Episode #4.3 - S4-E4
Revealing mistake: When Janice is speaking to Johnny by the pool, it is pouring down with rain. Look at all the parasols/walls etc in the background and you can see the sun shining on everything. The rain is artificial rain.
