After saving a ruby file it is parsed and syntax errors are displayed. The errors show up in the vertical ruler on the left side of the editor and in the Problems View.

Figure 3.5. Syntax errors in the problem view

The error position is often not very exact, which results in long red underlings of text in the editor. The underlining can be switched off in the preferences: right click on the vertical ruler on the left side of the editor and select Preferences.... The screenshot below shows the settings for the error annotations. In the upper right part deselect Text as squiggles.

Figure 3.6. Configure error annotations