For me, it was always a memory issue

Windows didn't have enough free memory to correctly draw the screen

Closing down everything, then starting afresh with just Notes client & designer fixed it

Do you have any "on error goto xxx" lines? Look for any colons in this line (or any goto line). The colons are permissible syntax, but the debugger or the compiler misunderstands them and misfits their line numbers.

There's another issue: watch for blank lines mismatching at the start of each function call. Characters (tabs, spaces, sometimes it's just lines) added outside any function, but not commented out, can throw off the line numbers in debug. Sometimes you can find these issues by just watching where the code editor thinks it is -- its first line.