If you're working flat on the circular needles, the mistake probably happened when you either skipped a row or worked a row twice. The extra row throws all the rest off.
When you know you are working correctly, take a good look at your work to help you recognize the rs and ws so you can catch mistakes before you get too far on the row.
So many patterns use knit for the rs and purl for the ws. This can be your clue.
Do you have a link to the pattern?
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry