PayPal has on several occations used it's near monopoly power to harm projects it for some reason does not like. It does this by witholding funds from the project's accounts, often with no explanation or any accountability. In cases where the blocked project is able to raise enough media attention, PayPal will reopen the accounts and pretend like nothing happened. Again usually without any explanation.
On this page I'll try to document the cases I know about to have it as a raference for later.
In June 2016 PayPal closed the account of cloud file syncing service Seafile and demanded that they would have to monitor their users traffic and content of the files they uploaded to the service. After negative media attention, PayPal reopened the account, but Seafile wisely chose to move to other payment providers.
In October 2011 PayPal froze the account held by Diaspora, a free, decentraliced and self-hostable social networking platform. After significant media attention and pushback from the Diaspora community, they reverted their decision and apologized to the project.
In August 2010 PayPal froze the account of Markus Persson the original developer of MineCraft. Another article about the incident.
In September 2005 PayPal froze the account created by the Something Aweful website for collecting donation to the Red Cross for helping victims around New Orleans after Hurricane Katrina hit the area. The money was eventually returned to their donors.