Yes, I had some trouble with some Ulster 50 Pound notes. I rushed to buy some currency as I was leaving a Saudi airport, (better rates than in the UK) and received 240 pounds.
A couple of days later I used a 50 note to buy some shoes (better quality). The assistant was gone a long time the said that she had to get pemission to accept the note. I looked at the others and only then realised that they were all from NI.

I had to rush to Gatwick to meet an incoming flight and changed the remaining notes back to "Bank of England" at one of the banks there.