If you know the couple really well, you could be tempted to try to joke around or tease them about something that's somewhat of an inside joke or a story not everyone knows (or more importantly, one that not everyone should know). Plenty can go wrong when trying to pull this off.
1. If you're writing it down, remember that it may not have the same tone when read as the tone you're trying to convey in the message.
2. If you're being recorded, you could more easily get your tone across, but be sure you don't say something you shouldn't. It can never be taken back.
3. If you're the best man, don't joke about something the groom did prior to the wedding day--even if it was years before he met his bride-to-be--that could be hurtful to his new wife.
4. If you're being recorded during the reception and you've already had a glass of wine or two, your safest bet is to say something simple like, "I love you guys. You're the most perfect couple. You'll have a beautiful life together."
Remember, you can always send your funny "best wishes" message in a card later if you still feel it's appropriate.
