I flew from Stansted to Knock with Ryanair. As the reason for my visit to Ireland was to measure up some industrial equipment I carried with me a telescopic measuring stick. In collapsed form this was approximately 20mm x 30mm x1metre. I presented it at Stansted as hand luggage, It was approved as such so everything was OK. The flight was completed and off I went to the factory.
Upon my return the following day, I was refused entry to the flight if my "hand luggage" was to accompany me. It had to be entered as hold baggage. When I asked why, I was told "You could use it as a weapon." I complained that no such supposition had been made the previous day at Stansted. My protestations fell on the deaf ears of the teenager who was checking me in. I asked to speak to the supervisor. "There is no supervisor here, she is at lunch!" If I wanted to speak to the supervisor I was welcome to wait until her return. By which time the flight would have left. Fait accompli.
I was allowed to hand my precious implement to the baggage handler actually loading the hold and needless to say the thing was (only slightly) damaged when I saw it again at Stansted.
As it happens that was the last time I flew Ryanair. Adding injury to insult I found later that they had charged my credit card twice for the same flight. I never got my money back.