Way back when I was a new PPL(A) in my first summer of flying, a friend and I flew from Welshpool to Blackpool for the day. The TAFs wer givng PROB 30 TS, but it was a lovely day, and no-one expected the TS to amount to much. We had lunch etc, and set out to leave in late afternoon. We were told there was thunderstorm activity, but again, no-one seemed to think it was that bad. Well, before reaching the Liverpool area we reached a black wall of cloud, heard various other pilots calling that they were diverting or returning to Blackpool, and we made it back there 5 minutes befiore a massive storm hit, with pink forked lightning and almost non-stop thunder. Blackpool was awash with people on mobile phones reorganising their lives, there was an impenetrable line of TS from Liverpool to Leeds, and virtually everyone was stuck overnight. We eventually made it back to Welshpool the following morning.
Since then I carry a toothbrush and spare underwear in my flight back, and treat PROB 30 TS with a great deal of respect.