I often see people who have had too dark a tan for their skin type. It's sometimes a combination of either the client who wants to look the darkest they possibly can or the spray tanner not having enough skill or the right product range to cater for all skin types.
When you are/have been sprayed too dark you often suffer from poor fade results. This is because the darker the spray tan (ie solution DHA strength) the more it dries out your skin leading to premature exfoliation and loss of colour. A prime example of this is when you get "lizzard skin" where the tan tends to lie in the cracks of your skin. The best thing to do with this is get rid of it by thoroughly removing your old tan before re-applying a new layer. People who tend to tan consistently will often still have remnants of their old spray tan on their skin and will then be sprayed over the top. Eventually you'll suffer from build up and your skin will just look really dried out. The biggest risk, is of course, your client will look orange - something that we need to eradicate in the spray tanning world.
Every spray tanner should analyse their clients skin to determine their natural colouring. From this they should advise the client which colour will be the best fit for them. I find that when a client insists they want to be the "darkest possible" and they are fair skinned, I explain to them the pitfalls and they generally listen and go with my advice. Here's an easy to follow infographic for you to use to assess your clients skin and which solution strength you should use. Above all you should never rely on one solution strength being suitable for all your clients - it's simply not professional nor possible.