Just Book 7?
I can think of 10 right off the bat!

Falling into the protective charms of Tonks parents house
The Minister giving bequithed items (legacy's from Dumbledores death) to Harry on the last night at the Burrow
Shakelbolts Talking patronus ...Just in time
Hermione having a bag of trickys all ready ... with cloak and ALL
Kreachers pain revealed ...(to find Mundungus)

etctetcetc ...but the one I thought was really lucky !!!!!

Being in the same forest as a group of travellers who, stopped for water at exactly that point in the stream where they could talk and reveal news from a far that could be heard through the Muffliato charm (So ...you can here them ...but they can't hear you! Now that's lucky ......

or is that oober fantasmagorical ingenious?