Personal bug bears would include in no particular order, crocosmia, lucifer is right as I used to scream to hell most times weeding in my old place. I DO like it, but it'll be in a trough if I have it here, under lock and key! Sweet william, forgetmenots and violas. My mum helps out in my garden, and she just can't compost anything!!! I'm trying to grow something on in pots and there's 3 inch deep of this gubbins smothering it!! All of it self seeded and unwanted, but try telling her that! I'm quite fortunate that I usually read up on something before planting if I've been on a mad impulse buy. Table top sales and boot sales are the usual culprits for things that take over.
I know what people mean about aquilegia too, another that self seeds ten a penny in the wrong place, yet in a heated propagator take an age to come up, and then are tiny seedlings! I too have sycamore every year thousands of the sods! I also have a next door neighbour who has a hazel that overhangs, fortunately I've neglected to tell them it makes the nuts on this years growth so as it isn't pruned significantly less hassle last year! Mind, I still have seedlings trying to survive in my lawn, and that's being cut every week and feed and weed and lawn weeder spray !! Goose grass? (sticky bobs) it grows everywhere on my neighbours gravel, the local wildlife and cats ensure I enjoy it too!
Oh and clematis tangutica, lovely climber, I gave a seedling I grew to my parents who planted it at their static caravan against an arch. The site was next to a river that flooded regularly. The outdoor garden bit was gravel, when I went there the next year, to my amazement you couldn't see the gravel for tangutica seedlings, all about a foot high, literally 100 a square foot!
It's funny though that what I struggle to grow is a menace elsewhere, I'll try to keep it in mind. Afterall, if everything grew easy, now where would the fun be in that?