Paperclip attachments can collide across development, test, and parallel test processes when they share one storage tree. Separate environment-specific paths prevent overwritten files and missing test data.

Nginx returns 413 Request Entity Too Large when client_max_body_size is below the upload size limit, often during file uploads.

Paperclip attachment checks can give false positives because the attribute returns an attachment object. Use exists? to test whether a file is actually attached.

Insert several empty rows in Calc at once by selecting multiple row headers and using Insert Rows; the new rows inherit the style from above.

Compute readable differences between two strings in Ruby or Rails with the differ gem, including word, line, and character comparisons plus HTML or color output.

Rails applications can answer browser preflight requests and allow controlled cross-origin access by handling OPTIONS requests with CORS headers.

Browser requests across domains are blocked by the same-origin policy unless a service allows them with CORS; it replaces JSONP and supports regular XMLHttpRequest and non-GET methods.

Cross-origin web pages cannot normally communicate, but <script> requests can pull executable JavaScript around same-origin limits. JSONP uses this loophole to load dynamic data from other origins.

Web application developers increasingly need operational knowledge beyond coding, testing, and deploying. Traditional development workflows now overlap with running and maintaining production systems.

Capybara matchers can be applied to standalone HTML fragments or strings, not only full pages, making fragment assertions easier in test suites.

Create ZIP archives in Ruby with zipruby; add existing files, string buffers, and directories using Ruby bindings for libzip.

Serving files as downloads in Ruby on Rails uses send_file; it returns the file directly and overrides the normal render response.

Keep the cursor on its row while scrolling a Vim buffer, avoiding jumps to the top or bottom of the screen. Default keys move the viewport by a line or half screen.

Sets Thunderbird archive folder date formatting and folder granularity for year and month subfolders, and can modify the A shortcut with Shift, Control, or Alt.

Run Selenium tests in a hidden X session with vncserver, keeping the browser accessible for inspection without adding a gem.

Passenger can start a Rails app in the wrong environment unless Apache is configured with the correct RailsEnv or RackEnv setting.

Firefox 3.6 print output cuts off long tables, often due to CSS floats, overflow rules, or nearby h1 and caption tags.

Hidden X displays can be inspected by taking a screenshot of the active display, including Xvfb-backed sessions used for headless tests.

Rough JavaScript equivalent of Rails simple_format for turning plain text into HTML paragraphs and line breaks. Whitespace preservation is not included.

Manual queue processing in Resque specs avoids relying on an external worker while keeping behavior close to real job execution.

Resque dashboard access can bypass ApplicationController filters in Rails 3, so a Rack middleware can protect it with Clearance and a remember token.

Ruby 1.9 upgrades in Rails apps often fail because of gem incompatibilities, lambda arity issues, and encoding mismatches between files, strings, and databases.

File.chmod sets Unix file permissions in Ruby, where the leading zero matters for octal mode values.