I saw Columbo goes to College yesterday and he talked about a tooth-print on a chewing gum what he searched in a basket to compare with a tooth-print on a piece of cheese in a former crime...
there was an epsidode with the cheese but I don't remember the chewing gum thing...
The cheese bite mark is a reference to one of the Patrick McGoohan episodes - Ashes to Ashes I think (I'd check in the episode guide - but that seems like cheating.)
It was Agenda for murder for clarification purposes.
I always thought that due to the mention of columbo sloving the murder on board in troubled waters that A matter of honour was the very next episode (columbo gets off boat and word has spread) and therefore was suprised to find a while back that they are from different seasons.
And the first time he ever refers to a previous case is in Double Exposure...he comes in and says he was working late on that Hayward case....referring to Candidate for Crime.