Oliver tells their therapist that it was 10 weeks and 4 days between when he and Shane first said 'I love you' and got married. But both Norman and Rita and Oliver and Shane got married on Saturdays and Oliver first told Shane he loved her the day before Norman and Rita's wedding so it couldn't be 10 weeks and 4 days.
When Oliver was showing Shane all the items he saved that she made special the napkin she drew on to show what jazz looked like to her wasn't the same. The original Oliver folded and kept in his pocket and this one didn't have creases from the folds. The drawing was also a little different than the original.
Joe tells Charlie that one of the Dark of Night Awards displayed in the DLO is his. But the stand with 5 Dark of Night awards has always had the 4 POstable's awards and Oliver's grandfather's award. Joe's was never with theirs in the past and there isn't a 6th cup there now.
The objects Oliver kept that made him think of Shane included "the last rose of summer" but the rose shown is orange and the roses he gave Shane were yellow.
When Oliver trips over Shane's shoes in their hotel room he kicks one landing it close to the other shoe. Then the camera switches to a wide shot and the shoes are far apart again when Oliver picks them up.
When Oliver is reading to Shane from 1 Corinthians 13 he skips part of a verse. The Bible says "...love bears all things, believes all things, hopes all things, endures all things." There is no version of the Bible that leaves out "believes all things". And while it's common to skip over whole verses when reading out loud it's not common to leave out just a part of a verse the way Oliver did.
After Shane tells Oliver she is pregnant he pulls her close and kisses her but then holds her at arms's length to ask her again if she's sure. When the camera changes angles when the clock chimes Oliver is holding Shane close but there wasn't enough time for him to pull her close again.