ETHEL MERTZ -- TROUBLEMAKER!
As everyone knows, Ethel is usually Lucy's (often reluctant/unwilling) patsy and cohort in mischief, but I've noticed there were a few times when Ethel caused problems/made trouble for Lucy and Ricky on her own. Here are a couple of examples:
In a early episode a gossip columnist writes a blurb about Ricky alleging that he has been "making Cat's Eyes" at a pretty dancer in his nightclub act. Lucy originally thinks nothing of it, rightly concluding that it's a just a publicity stunt meant to keep Ricky's name before the public. That is, until Ethel comes over and brings it up and persuades Lucy that Ricky is cheating on her.;
In the first episode of the Trip to Europe story arc, Ricky tells Lucy that he can't afford to take her on this trip, that it will be very brief, that he won't have time for sightseeing, etc. Lucy is initially insistent that Ricky take her along, but she relents after Ricky promises he'll take her on an extensive trip to Europe next year. As Lucy says, Ricky has never broken a promise to her.
It all seems settled until Ethel appears and persuades Lucy that Ricky has suckered her into believing the trip won't be worthwhile for her so he can spend all his time partying with the boys in the band, and possibly cheating on her. As Ethel says, "Oh Boy, did he ever pull the wool over your eyes!" This leads to Lucy refusing to speak to Ricky and going on strike.
Any others?