I live in the UK, and in the first of two elections I will be participating this year (local government), there will be a single transferable vote system. I get that I should probably rank all the candidates, but what happens if I get a bit confused and miss a number (for example 1,2,4,5,6) or give two candidates the same number (1, 2, 3, 3, 4)?
This is a matter for the judgement of the returning officer.
The key principles are given by the electoral commission
3.20 The decision of the (Acting) Returning Officer is final, subject only to review on election petition.
3.21 Establishing voter intention is crucial when determining doubtful ballot papers.
Where there is a clear intention apparent, for example marking 1,2,4,5,6, the ballot should be counted.
If the ballot is marked 1,2,3,3,4, the returning officer should treat this as a clear indication to vote for the first candidate, and initially count it as such. If the election comes down to a choice between the two candidates marked "3" the returning officer should reject the ballot at that stage, as no voter intention can be determined.
However, if you notice that you have made an error in filling out your ballot, you should ask for new paper, which is within your rights.