Sometimes after changing content in contentful you may get a TypeError: Cannot read property 'internal' of undefined error. The only fix I've found for this is to unpublish all contentful component pages, and republish them ~4 at a time, building each time. Relevant issue here:
contentful/rich-text#53
gatsbyjs/gatsby#6949
Embedded entries don't resolve when using gatsby develop: contentful/rich-text#46
gatsbyjs/gatsby#10592
gatsbyjs/gatsby#9821
Nested embedded entries are not resolving even when using gatsby build gatsbyjs/gatsby#10592 (comment) (EDIT: this seems to fixed, keeping here for reference)
When updating gatsby, some entries do not resolve. Don't update gatsby until this issue is fixed: gatsbyjs/gatsby#10592 (comment)
When using gatsby build, the CSS is not output correctly. Seems that extra CSS from large scale components is being brought in. This is due to the webpack plugin importing require()'ed css even when the code isn't run (in react-spectrum Provider component). gatsbyjs/gatsby#9911 gatsbyjs/gatsby#10706
Some images are not at 2x currently. Need to spend time looking at gatsby-image and contentful image API and our ResponsiveImage component. Issue here: gatsbyjs/gatsby#10987
Other relevant issues: contentful/rich-text#54 gatsbyjs/gatsby#10403 gatsbyjs/gatsby#4610