Created attachment 6730 mousepad-close-button.c patch resolving the issue Icon for "Close this tab" button fails to load. However, same icon ("gtk-quit") loads just fine in "Close tab" button in File menu. Bug occurs on mousepad 0.4.0 on Arch Linux with mousepad compiled against gtk2 (2.24.30). Identical behaviour occurs when recompiling against gtk3 (3.20.6). I'm not sure whether bug is strictly Arch-based or is reproducable on other distros. I send as an attachment a patch for mousepad-close-button.c which seems to resolve issue for me.
Patch looks good to me.
Couldn't reproduce on Xubuntu 16.04 and Archbang, it appears this is an isolated case.