I do so agree regarding Spanish bluebells.  They are the only plant coming up all over my new small back garden which had been grassed over and are also pushing through the gravel at the front!
I am very late to this thread so apologies if this has already been said. DON'T PLANT MUSCARI (grape hyacinths). We "inherited " them in our front garden, I have spent far too much time in the past 20 years trying to reduce them to a manageable level . We gave up trying to get rid of them long ago. ☹
Oh dear @floralies, that sounds painful. How many more of us will have gardening related injuries?
Lots I suspect. Don't cut hose pipe with a very sharp folding knife. It will stick to the hose and then fold round your finger. It doesn't start to bleed properly until you unfold the knife, then it really gets going!
Don't prune a huge ivy hedge without gloves on if it's full of sharp wire. You might nearly slice your finger and bleed all over the carpet and up the stairs. Know your hedge, take it gently. Or keep a first aid box by the fridge.
My personal 'don't' (as a mental note to self): don't take loppers to the garden when in a bad mood. You might wake up the next day and find out you lopped out half the garden - trees down, bushes gone, half the perennials pulled out, like a crazed, highly caffeinated Kondo has gone to work.
Don't plant lemon balm unless you want the garden to be full of lemon balm. The damned stuff manages to self seed at quite a distance from the original plant. Ditto lunaria.
Don't be put off by the fact that nothing wants to grow in part of the garden. You will eventually find something that likes the soil (lemon balm probably).
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DON'T PLANT MUSCARI (grape hyacinths). We "inherited " them in our front garden, I have spent far too much time in the past 20 years trying to reduce them to a manageable level . We gave up trying to get rid of them long ago. ☹
Lots I suspect. Don't cut hose pipe with a very sharp folding knife. It will stick to the hose and then fold round your finger. It doesn't start to bleed properly until you unfold the knife, then it really gets going!