I suppose when you get right down to it, we could critique almost every episode for some flaw. But paying attention to small details like Fred noted to is fun too I, in particular, found it very amusing when Columbo said to Abagail Mitchell "It's true, I'm in the market for a new pair of shoes but they aren't my size." Things like this are what makes Columbo so unique. you wouldn't find that kind of humor in Law & Order or other contemporary mysteries.
The drug put in Marcie's coffee was the chemical that makes you abhor alcohol. Creighton drugged the champagne so whoever would drink it would fall asleep. Creighton wanted the boyfriend to drink it but not Marcie. He wanted boyfriend asleep so he could strangle Marcie and make it look like a struggle.
Not sure about the truck. Would have to see it again.