In the 2007 Scottish Parliament elections, which operates under the Additional Members System (AMS), the ballots for the regional and constituency votes were combined onto one piece of paper, as below. This marked a change from the previous elections in 2003, where the ballots were split onto separate pieces of paper.
Many voters were confused by this new arrangement and either ignored one side of the ballot paper, used two votes on each ballot, or both votes on one ballot or similar. The elections also took place in conjunction with the Scottish local elections, which switched to Single Transferable Vote (STV) that year - meaning voters had to number candidates in order of preference. Although this took place on a separate ballot paper, some voters tried to number candidates in order of preference on the parliamentary ballot as well. The Electoral Commission produced an independent review into the elections which was published in October of 2007. The images in this answer are borrowed from said review.
There were a large proportion of rejected ballot papers - with roughly 3% of the regional ballots and 4% of the constituency ballots being rejected nationwide, and in some constituencies a rejection rate of over 10%. A breakdown of the reasons for rejecting ballots is shown above.
In particular, in two regions, Glasgow and Lothians, there were a large number of candidates for the regional ballot, which meant a redesigned ballot paper was used to avoid changing the size of paper. This redesign removed the directional arrows and had abbreviated instructions at the top of the paper, which the Electoral Commission's review found may have contributed to that area's high rejection rate - although it also proposed that the high level of social deprivation could also go some way to explaining this incongruence.
The review recommended that in the future, the regional and constituency ballot papers should be separated, and there should be a "greater focus on contingency plans to ensure the number of political parties and candidates can be accommodated".