At BBO you can set your own skill level in your profile. Many people just put "expert" or even "world class" while they can't play a single card right. Yesterday I encountered a "world class" on this hand:
We were bidding quite agressively:
1♦ - pass - pass - Dbl
pass - 2♠ - pass - 3♠
pass - 4♠ - pass - pass
Our WC needed a long time before he passed this out. I think he wanted to double... GIB says the contract can only make after a ♣ lead (which is very unlikely), otherwise it's always -1.
So LHO starts with a small ♦ which immediately takes away an entry to set up ♣s. Still they are our source of tricks, so after taking ♦A I immediately play a low ♣ to the Queen. LHO hesitates a moment but plays low. Now I cash ♠AK and play a small ♣ to my T which loses to the Jack. LHO correctly continues ♦, but a ♥ would also have worked. I ruff, and play ♣K, covered by the Ace which I ruff in dummy. Now we get our world class action: RHO overruffs without a thought! 4♠= and 12 imps for the good guys.
All our WC player had to do is count some HCP and some trumps, which is the biggest step from beginner to intermediate. If he would've done that, he'd know that I can only have ♠J in my hand, without another trump. So that's the entry to my hand. If you think for a second you'll realise that you need to discard and ruff the next ♣, just in case I have 5 of them (why would I accept the invitation with 8HCP?). There's no way to my hand anymore, I can only discard 1 ♥ in dummy, and you still have ♦ for exit cards. I'm going down -1 for sure.