When Muriel hands her father a check at the end of the film, you can see it's for $5,000. She says, "That's not all of it..." and in fact it isn't, because the amount was established earlier as $12,000.
When Betty Heslop writes the first check to "cash" for Muriel to buy cosmetics, it is dated 12-1-1994. When Muriel hands her father a check for $5,000 near the end of the film, that check is dated 2-11-1992.
While Muriel and Rhonda are imitating ABBA on stage, Tania and Nicole get into a knockdown fight and the entire table is disrupted. When we cut back to Muriel and Rhonda, the girls still sitting there just watching. We can see the backs of their heads, and their dates are standing enjoying the show as before.
When Muriel is trying on a wedding dress in the shop, and is admiring it in the full-length mirror, to the right in the mirror, you can clearly see a crewmember standing there, before quickly darting out of the reflection.