We went to Barcelona at the end of April, and purchased tickets in advance from home for the Sagrada Familia. When we got there, we were told that our tickets had been cancelled. We had to rebook at a later time that day. Be sure and print out your tickets if you book in advance, and make sure that the funds for the tickets have been taken from your bank account or charged.
When we tried to visit Park Guell mid-morning, we were told that they were sold out for that day. We didn't know that we needed to buy tickets in advance for that park, since on their website, it just mentioned that reservations were "recommended." We didn't think that April would be a problem.