Developers increasingly prioritize accessibility and usability to create inclusive web applications, yet their discussions often reveal misunderstandings between these concepts. We aim to investigate the extent to which developers misclassify or conflate usability and accessibility in Stack Overflow discussions. Our study analyzes 336 posts tagged with both usability and accessibility using a mixed-methods content analysis approach. The result shows that 12.2% of the discussions contain notable confusion between these concepts, particularly at the system level (34.5%), in comparison to feature-level issues (9.9%). The distinction between usability and accessibility becomes particularly challenging when developers discuss broader design decisions. These confusions include the misclassification of issues between the two concepts or the failure to distinguish between them. Our findings show that developers did not make explicit references to usability guidelines in their discussions, while only 20.2% of accessibility-related posts acknowledged standards such as WCAG, highlighting a significant gap between formal standards and their practical application in development contexts.