Autism and ADHD are a such a good example for this. While 15 years ago the DSM still said that it is impossible to have both, today we know that there is a huge overlap. Depending on the studies, when you have Autism, there is a 40-70% chance that you also have ADHD. And if you are diagnosed with ADHD, the chances to also have autism are between 20-50%

And there are enough other common comorbidities that could be seen as the "main" diagnosis and prevent you from being correctly diagnosed, like depression, odd, ocd or anxiety.

It's so important to get a psychiatrist that is specialized and it's a shame that it seems to be an almost global problem to even get any.