‘Eighteen months after his release, he was caught red-handed taking part in an armed robbery at a bank in North London.’ This unfortunate individual was found doing something clearly illegal and criminals who are caught in the act have little chance of escaping punishment. Clearly, the person in question was caught by surprise but you can also catch someone at a bad moment, as in 'I'm afraid you've caught me at a bad moment. Could you call back later?' Saying this is often an effective way of deterring (or at least delaying) cold callers on the telephone.
If, on the other hand, someone is caught off (their) guard, something happens that they are not ready for, as in 'Local residents were caught off guard by the floods and had to be evacuated from their homes by boat'.
If you catch a glimpse of someone, you see them for a very short time, as in ‘People lined the streets outside the theatre, hoping to catch a glimpse of her’. You can also smell something for a very short time, as in ‘He caught a whiff of expensive perfume as she brushed past him'.
Of course, you can catch various diseases, particularly during the winter months, and you might be warned against catching your death (of cold), as in 'Get out of those wet clothes or you'll catch your death of cold'.
Perhaps the best antidote to the winter blues is to go somewhere warm and catch some rays (go sunbathing) but don't overdo it or your friends might say ‘You’ve caught the sun’, meaning that you have painful red skin after sunbathing for too long.