can bitcoins somehow end up being lost in all this ?
tweet gets somewhere unwanted,
or receiver does't know what it is,
or just forgets about it, discards it, never collects and so on ?
Ehhh perhaps you aren't understanding the way it works =) The Bitcoins are deposited by someone who creates the campaign. When users retweet the campaign, they're paid from those bitcoins based on the bitcoin address or email they've entered. If it's not a valid BTC address, the coins aren't sent. If it's the wrong email, coins will sit in BTCinch's system until they're claimed.
If a user sends btc to a new address or email and never does anything with them, then I suppose those coins are gone forever. But that doesn't hurt anyone except the person who forgot about them - all other coins become more scarce and thus more valuable in proportion.